Compact Cranks – The bottom line?

The only thing I had to do when I switched to a compact was to shorten my chain.

I don't know about different levels of shimano compacts - I can't imagine the difference between ultegra and dura ace in a crank would be much more than a slight weight difference.

You can read a bunch of nattering about front derailleur height, even compact specific FDs.  Not necessary IMO.  I've put compacts back and forth on two bikes and gotten it to work well without any front derailleur changes or even chain length changes.

Since you can't adjust your front derailleur's height, just check to make sure you don't have a rubbing chain on the bottom of the derailleur's link when in the small ring.  You might have to toe it in or out slightly. 

I recently put a compact onto my 1996 litespeed ultegra - I changed the BB, moved the FD down and shortened the chain. The whole operation took about an hour. Specialized tools required were BB removal tool and crank puller. (edit: and chain tool of course)

In moving from a regular double to a compact double, Assuming that the BB is the same type, all you are going to need to do is

pull old crank

Put on new crank

shorten chain a few links

adjust deraileur height (maybe tweak your limit screws a little)

that ought to do it

Only thing I can help with is Shimano only makes one level of compact cranks. They place it in between Ultegra and Dur-Ace. My two new bikes are Ultegra and I had to pay a small amount extra for the Shimano compact crank when I bought my Tarmac.

Typical chainrings on a compact are 50/34

a 50/11 is about the same gear as a 53/12 (actually a little higher). A 34/27 is about the same gear as a 30/25.  If you are concerned about having a low enough gear for climbing, a 12/27 cassette on a compact gives you almost the same low gear as a triple. If you are concerned about top end a 11/23 cassette will give you the same gear spread as a 53/39 with a 12-27.

A compact double will shift better and weigh less than a triple.

Are you rocking a 55 on 700c wheels?...that's some big meat.

Others have mentioned weight, you will save some, but maybe 100 grams tops.   Pick up a spare 30T ring sometime.  It is insanely light.    I think the biggest reason is that people prefer the shifting response on a double, but like the gear range of a triple.   The compact cranks puts you somewhere in the middle.   Plus, if you bike came with a double, but you need lower gears it is more $$ to go to a triple.   You would need a triple deraileur and a new triple left shifter along with the new crank.   With a compact you just need the new crankset.