What's nice about it compared to the armory itself is that it allows all of your raid team members to be viewed in the same place at the same time, and will also instantly let you know if your raid members have full gems and enchants. If you want to analyze a team member in greater depth, you can click on their name and it will display that character in a manner similar to the armory, as well as letting you know if that character "passes" the gem, enchant, and talent audits.
While all this functionality is very nice on the surface, it is a new site and there's quite a bit yet to be hammered out before it's truly useable. Of course, the usual limits apply to data that's pulling from the Armory: if your raider is in an off-spec or non-raid gear, you're not going to get an accurate picture of their raid set-up. I know that I, personally, am terrible for this (sorry raid leader...) because I am only ever in my healing spec and gear if I'm raiding. All other times I'm in my soloing DPS spec, so that's usually how my toon appears on the Armory.
I pulled my own data while testing Whozawhat, and as expected, all my gear and enchants are feral spec, which tells my raid leader nothing about my actual, resto raid readiness. More importantly, when you click on an individual character to go to the character audit page, their gear and stats are supposed to display upon mouseover the way they do on the Armory, but right now this doesn't happen. The tooltip just says it's loading. According to the webmaster this is a known bug and it's being worked on, but it's a big one.
Overall, Whozawhat is a tool that definitely has some potential and could really benefit from users who bring feedback and suggestions to the table. If that's the kind of thing you're interested in, you may want to check it out.