I received an email four days after he received the manuscript. I was positive it was a rejection (since he got it via a referral and hadn't read any sample pages) and that he hated it. The first paragraph detailed the things he loved and how excited he was about the manuscript. I expected a "But..." in the next paragraph. Instead, I got "At this point, I'd like to extend an offer of representation."
After I reread the sentence six times, I screamed for my roommate and made her read it and verify I wasn't dreaming. He also had some edit suggestions in the same email--some I agreed with, another I didn't, and he extended an offer to chat on the phone.
For me, the first thing I did was email back, thank him for the offer, and set up a time to talk on the phone. I also let him know I had to contact the other agents currently with material and let them know about the offer. If you have material with other agents, you always want to give them the courtesy of informing them of an offer and giving them time to decide, as well.
Talk to the prospective agent before you agree to let them rep you. Talk about their agenting style, talk about your career plans and see what they think, ask lots of questions. Sometimes it's so exciting to get an offer that you don't do the right prep and you end up with an agent you just can't work with.