I've got both an Express and an Extreme at home. I use the express to stream music into the kitchen/dining room and have the Extreme in the living room along with my Thomson modem. I used to have the extreme upstairs in the study but moved everything about to make the most of the available gigabit ports I needed in the study. According to the Network Utility my MBP connects at 130Mbit/s and without getting too carried away with benchmarks, I feel I get excellent throughput. My MBP is the only 5GHz N device I have, so everything else (phones, iTouch, Squeezebox, other Airport) connects at 2.4GHz.

I've not used the USB hard drive feature of the Extreme as I already use my ReadyNAS for stuff like that, although I've read it's not particularly speedy. For what I was doing I didn't think I needed a Time Capsule and also the fact it's only a single disc seems a little risky to me.