Originally Posted by rschleicher

The pluses of the new 105 (5700 series), versus the prior 105 (5600) are:

- cables to the derailleurs are now "hidden" under the bar tape, along with the brake cables (just as they are with the current Ultegra and Dura-Ace, not to mention the SRAM groups) (whereas the 5600 had the derailleur cables coming out the side). This is almost entirely a "looks" issue.
- the pivot points for braking in both the new brifter levers and in the new 105 brakes themselves have been moved a bit - supposedly to provide a bit more braking leverage, especially when braking from the brake-hood tops. I don't know how noticeable this is. It's also not clear to me if the change is mostly in the brifters, or mostly in the brakes (I don't know if your proposed bike has the "full" 105 group, including the brakes, crank, etc., or if some parts are from other companies.)
- a small weight reduction - but as this is only 30 gm (about 1 ounce) over the whole group, it is essentially nothing.
- the new "short cage" rear derailleur (for use with a double or compact double) has a bit more "reach" than before, so that you can fully cross-chain, even with a compact double up front (16-tooth spread), and with an 11-28 cassette (17-tooth spread). In other words, the total "tooth differential" that works with the new RD is 33 teeth. (The old 105's figure was 29 teeth, which meant a 12-25 cassette.) Note that you can still use cassettes with a larger spread with the old 105 RD - you just have to avoid one or two of the worst-case cross-chaining combinations (which most people would avoid anyway). (Actually, the largest rear cog that the old 105 RD would support was a 27 - now it works with a 28.)

Sorry if the above is a bit complicated. The improvements in the new 105 are not THAT significant, but makes it a bit closer in functionality to the current Ultegra. Probably not enough of a difference to reject getting a 2010 model that didn't have it, but all else being equal, you might as well get the latest....