I used Disney because they came with the package. No extra fees or trying to come up with $500-$600 extra for an outside photographer. I didn't have to worry about coordination and I didn't have to worry if they were available.
I paid $225 upfront for a copy of all my proofs and as part of the package (it has changed since then) I also got a beautiful white leather photo album with (I think) 15 photos in it. At the time, the package also included a low-res CD of all my pics. I was married in 2005, next year (3 years) I can purchase the hi-res CD (and copyright) of my photos for $99. Unless things have changed, if you are a UK bride you will get the hi-res CD as part of your package.
So, all told, I will have spent $325 on my photos. I think Disney did an outstanding job for the price. My only regret was not asking them to take more pics.
Disney photographers are professionals. The quality of their photos are not less than any other professional. However, there may be a difference in style. There are many photographers on staff and they have a variety of styles (from photojournalistic to "posed"). Lately, I have heard that you can request a specific photographer and they will try to honor your request and several brides have been pleased with Ty, although I know there are a few others who's names I have heard.
Regardless of the photographer you choose, it is important that you show them samples of things you like. Make a list of shots you must have, shots you'd like to have, and shots you really don't like. Print out copies of photos you see on the web or cut out ones you see in magazines and show them to your photographer. And be sure to ask any photographer about the copyright issues, make sure that you get a letter or something from them with your CD or you may still have problems getting reprints.
Good luck with your decision!