Is bike fit really that important?

Depends on how far you want to ride it.

Yes, there's more margin for error in getting a bike that "fits" than in being "fitted" to the bike, but get one that's too far off, and there isn't anything that can be done to make it work.

If the bike generally fits you, it's possible to switch out the stem, bars, seat, and adjust each of these when you are fitted.

I rode a bike that was not a good fit for me for about 9 months when I first started doing triathlon.  I bought a used bike off of a friend who is 4 inches taller than me.  It was terrible.  Not only was the bike too big, the geometry was all wrong for my body type.  

There are really two issues here, one is getting a bike that fits and the other is having a good professional fit after you buy the bike. Both are equally important.  Take the time to talk to some folks at your LBS that really know what they are doing.  When I bought my road bike my LBS owner took the time to get to know me, my goals, and took all of my measurements before starting to recommend bikes.  Then, he recommended 2 or 3 different bikes that were in my price range and let me take them out for decent test rides. After I chose a bike he spent over 2 hours on the fit.  

When it came time to upgrade to a tri bike a few years later he did the same thing.  I now have 2 bikes that I can comfortably ride all day.

How specific are you about how a suit fits?  You pick the right sized jacket (arm length / chest size) & pants (waist / inseam).  And the brand and even THAT particular suit style may not fit the same as another of the same brand.

And you still need tailoring.

Think about how many hours you plan on sitting on the saddle.

When I bought my bike a few years ago, the staff at the bike shop just watched me ride a few different sizes and finally one "looked" right and they sold me the bike.  Now keep in mind I had to drive an hour and a half to find any bike shop and this one was selling what I wanted at the price I was willing to pay.  Unfortunately after riding a little for 2 summers it didn't feel right and I drove an hour and a half in the other direction and found out I should have bought a bigger frame.  The fitter spent hours with me.  Changing seat height and angles and changing stems and moving the handlebars.  It's better now, but about 3-4 hours is all I can stand on it.  And the trainer is worse.  About an hour to an hour and a half on the trainer is all I can do because it's just not comfortable.  When I'm in the market for a new bike (when I have some money) I'm researching bike shops and if I have to drive 5 hours I'm getting a bike that fits!

jami

I can ride my tri bike for 5-6 hours without experiencing anything that you describe.  

I just got a bike fit from arguably the best bike fitter in Chicago earlier today.  It was (extremely) thorough and involved even more than I expected.

The fitting wasn't cheap and took 2.5 hours but it feels like a totally different bike now.  I was really off when I started.  got back on my trainer afterwards and the bike was both more comfortable and my power numbers were a bit higher.

I'm really (really) glad I finally went in and got everything done.  

I bought my Tri bike earlier this year in June and was initially fit by the LBS.  The setup was fairly conservative and I have since made some modifications to the seat height and the cockpit for a more agressive position.

The current setup feels great and other than some saddle issues (I don't think that is due to the fit, but rather due to the stock Felt saddle) I can ride in aero for hours.

Like the OP, I was always skeptical about the benefits of a professional "fit" and felt that I could make the appropriate adjustments and figure out my own personal setup.

However, after reading several online articles and posts on BT, the biggest issue for me is that you may be sacrificing power if you don't have the most optimum fit. 

As I start to ramp up training in the next few weeks, I will be scheduling a professional fit to make sure that what I think is optimal really is the best position.

Even if they confirm that my current setup is right, it will be money well spent in my mind as I would otherwise always have doubts that I was leaving watts on the table.

Yes, this is why you still get fit. A lot of frames will fit a lot of people, but you still need to get the size close enough. And as seen from the OP's later posts, the fit can really help to dial it in with the right size parts so that it works that much better.

Possibly, it depends on just what the issue is. If the fitter is good, they may be able find if it's something with the position or if you have some physical issues that may need to be addressed. Something is definitely going on though.

Curious though, is this on a trainer or outdoors? If both, is there a difference between the settings? Are you taking stand-up breaks? The trainer is usually a bit tougher on the rider with these things.

I was hoping I could be the first poster when I read the title so I could just answer:

"Yes."

I'm not sure why bike fit (especially for a tri bike) is even a topic of discussion.  It's absolutely mandatory as far as I'm concerned.  If one considers power and comfort to be important issues (and they're the two most important issues) then one should consider a good bike fit to be of paramount concern.

It's funny that people spend thousands of dollars on a bike and either 1) discount the importance of a bike fit and/or 2) balk at spending a couple of hundred bucks to get a proper fit.

The latest trend with the bike manufacturers and their latest generation of tri bikes has been to make them more middle of the road when it comes to fit.  Cervelo and Felt were always at one end of the spectrum and Cannondale was at the other end and Trek and Specialized were somewhere in between.  The newer bikes have more common geometries and more options with the integrated cockpits to fit just about everyone who isn't an extreme outlier when it comes to body size and/or proportionality.  The old adage of "find the bike that fits first" is less of an issue than it was a few years ago.  That stated, it's still important to know your fit parameters to make an informed decision on your bike of choice and what stem, bar, etc. get you into the best fit.  

Remember, the best fit isn't always the most powerful or the most comfortable.  It's the fit that balances the two in a way that works best for the individual.  I ride a relatively aggressive fit with a 14cm drop.  Most people probably ride with a drop half that if not less.  Newbies will usually have fits more on the conservative side with comfort as the primary concern.  Fits generally get a little more aggressive the more experienced one becomes in the sport.  Which brings up the point that bike fits tend to be dynamic in nature.  A fit that worked for you two years ago might not be optimal now.

Compromised handling at speeds in excess of 40mph isn't as big an issue when running.

I was pretty much in the same boat as you four years ago.  I assumed a 56cm bike was the same no matter the manufacturer and boy was I wrong.  I bought a bike that was completely wrong for me and could never get it fit right.  Like everything triathlon related in my life, I make a mistake and then research the heck out of it and become a quasi-expert in the topic as to not make the same mistake again. 

As I stated previously, the bike manufacturers are moving to more of a middle ground with the bike geometries in order to sell more bikes to more people.  This, first and foremost, is found in the super bikes for the big manufacturers (Cervelo P5, Trek Speed Concept, Specialized Shiv, Felt DA, etc.).  Lesser known brands (I think you mentioned you were looking at a Fuji) are a generation or so behind your Cervelos and Treks so their frames "may" still fall into the more conventional long & low or tall & narrow type geometries depending on the manufacturer.  This is my long winded way of saying that a 56cm (or size Large) from say Cervelo and Trek are more alike than they have ever been.