I haven't used mousekatrips and don't know anything about the services they provide but I have heard of them. According to their website, they've been in business since 2002. I ran across their website in 2006, when we were planning a big group trip to WDW.
They're based out of Utah - which isn't necessarily an issue. Buildabettermousetrip.com and mousesavers.com are both respected and have both been around for awhile and both of those Disney-focused companies are based out of Colorado. Smallworoldvacations.com utilizes a lot of home-based agents that are located throughout the country. And Lou Mongello of WDW Radio is from New Jersey.
I did a quick google search using the term "mousekatrips reviews" and it looks like they are Better Business Bureau members with a good rating. They're also Authorized Disney vacation planners.