Hey guys.
I'm an Arsenal fan, and write and moderate over at Arsenal Mania. Just thought I'd sign up over here to give you a Gooner's view of big Phil, and perhaps help you understand what it is you'll be getting.
Senderos has been a player that used to divide a lot of Arsenal fans. For a long time, many people thought he was the second coming of Tony Adams - both big guys, both good in the air, both with a lot of heart on the pitch, and both prone to making a lot of errors in the early years of their career. Tactically, he was one of the best defenders we have (though that's not saying much these days) - he may look clumsy, but he uses the ball efficiently out of defence, he holds the line pretty well, and he's good in the air.
However, he's also incredibly slow, and if a forward can get in behind him, he'll have a real hard job of making up the ground, and that is if he doesn't bring him down on the way through. He's a very smart guy (speaks something like 5 languages), is eloquent off the pitch, and vocal on it. However, he also really gives the impression of lacking mental toughness, and if something goes wrong (as it often does with Phil), a bit of an individual collapse is not out of the question. Oh, and whatever you do, if you play against Didier Drogba, don't even let Senderos in the stadium - he's stitched him up more times than a Women's Institute knitting class.
Overall, he's a talented guy, but not really tough upstairs, and that has cost him over the last few seasons. With a strong leader next to him, he could well flourish into an excellent defender (Jagielka maybe?), but all in all I was a little bit surprised that Moyes went for him.
Hope that helps.