I haven't been on the Utila Aggressor(s), but I was considering it for 2009 until reading the reviews in Undercurrent's "The Traveling Diver's Chapbook" (if you haven't heard of it, it's a yearly compilation of diver reviews of destinations all over the world from the subscribers to Undercurrent; the 2008 book is 426 pages long).
Mostly the crew got good reviews, but there were complaints about the shape of the boat from a number of reviewers - showers not working, showers leaking, air conditioning suited for a meat locker, bugs and ants, broken gearbox so running on one engine, generator problems, etc. A summary might be this quote from one reviewer - "The boat is the former Turks & Caicos Aggressor and should be sunk as a new wreck site."
Not sure which boat the Undercurrent reviews were talking about (I or II) so you might be on your own to figure out which they might have been talking about. As I said, what I'm relaying is just what's written in the Chapbook - I have nothing against the Aggressor fleet (I've done Belize and Caymans, and am on the Palau Aggressor in January 2009).
Also, reading the captain's logs for 2008 on the Aggressor site it appeared that the number of whale sharks they saw through October could be counted on one hand. That was my main interest in going there, so with an aging boat and a good chance of no whale sharks, I decided on a different trip (Palau Aggressor in Jan.).
Jim