Well 2 things...
If we assume power drain is generally linear across the capacity of a battery, is 15 much of a difference from 21? It should be about a 40% increase which is pretty noticeable IMO. Could definitely mean the difference between thinking about your battery all day and not worrying about it if the stock one is just barely getting you through with your normal use.
Personally however I'm waiting for this:
http://www.seidioonline.com/index.as...ROD&ProdID=888
Supposedly due out any day now. There is a definite size increase but I aim to use it on those days when I know I'll need a lot of battery time and swap the stock battery back in for typical use.