We also stayed in Sierra and loved it. We found out later (by seeing pictures posted here) that it does have bigger rooms than the other towers - just like the previous poster said. We did Goofy's dinner - it was OK, but we weren't crazy about it. It's pretty comparable to character dinner buffets at WDW, but I think it was quite a bit more expensive. We figured since it's the only dinner character meal at DL (I think this is still the case), that they must up the price due to that. I'd still recommend doing it, though, for the character interaction. We really liked the food at PCH Grill for Minnie's breakfast. Not as many characters, though, but great food. We also had dinner there (no characters) and really liked it. We did Hook's Pointe for breakfast - excellent.
We enjoyed the water show at the hotel. They also have waterfalls, koi fish, nice pools, shops...
This was our first trip, so we read a lot on the boards, read Birnbaum's, and visited the DL site. There were some great threads, so if you are able to do a search, that would probably give you even more info.
We had a great trip and absolutely loved the hotel.
Have a good time!