One thing you should know about protein skimmers is that the tank size ratings are all over the place. Good skimmers are closer to what they state in terms of what they can handle, but they are all over-rated.
As far as the skimmer you are looking at goes, I don't think it will handle a 90gallon tank for a variety of reasons. The pump is rather weak (317gph at 25W) and the "paddle wheel impeller" design that they tout is laughable (all it is is a dimpled version of a normal impeller). In the end, it won't take out enough to keep up with a normally stocked 90 gallon tank. Of course, you could always make up for that by doing more frequent water changes and supplementing with lots of activated carbon, etc. But, if you are like me, you'll want a skimmer that takes out so much that you don't have to make tank maintanence a second job.
Here is another good hang on skimmer that is reasonably priced:
http://www.marinedepot.com/md_viewItem.asp?idproduct=AQ1113
For $70 more than the Current Fission, you'll get more than twice the output.
I've heard that this dirt-cheap skimmer is actually fairly good:
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=13910&N=2004+113771
There is a guy on here that is always touting how great they are, wouldn't know myself. Maybe he'll chime in...
Finally, look around the Used Goods Forum. If you check there often enough you can get really good deals on really good equipment.