I bought the Ultegra 6700 version of this cassette. I didn't like not having the 16 tooth cog and the 17 tooth difference (11 to 28) made getting the chain length crucial with a short cage rear derailleur. So, I also bought the 12-25 and now I have a 12-28 that goes like this: 12, 13, 14, 15, 16, 17, 19, 21, 24, 28

If you don't miss the 16 tooth cog and/or you have a medium/long cage rear derailleur, you'll be fine. If you've got a short cage, be careful when setting the chain length. There isn't much room for error. Ie, set the chain length on the big-big combo and don't worry that the chain is a bit loose on the small-small combo.

Bob