Buy it.............. Great Value and can be the most influential addition to your ride. I ride a 50/34 compact with 11-23 cassette. This gives me the total range of a 12-27 on a 53/39 std crankset,
BUT............................. It's much more efficient because your gear ratios all the way through are much, much closer. It's more like having an Italian 6-speed in a Ferrari vs. a 4-speed. By managing your front derailleur properly, you can be in the right gear all the time.

It's fun to ride with friends and have the selection of 3 gears that are close enough together that any one of them will work. This aids accelleration and when climbing gives you the choice of more than a single cog. It will make you faster by giving you more options.

Also, ignore the comments about age, strength, etc. There are a few pros that run compacts in differing stages of the tour.(Bradley Wiggins for one) One size does't fit all