I went clipless a week or a week and a half ago. Like you, I'm finding soreness in muscles I didn't even know existed, although that's more or less gone away. My story is a bit different, though; I've been using toe clips for at least ten years. Suddenly, a couple months ago, I started getting this awful pain in the side of my left knee, so bad that I almost couldn't twist in and out of a car for the next few days. Turns out my foot was at just the wrong angle, and after cycling 75 miles like this, I probably gave myself a bout of tendinitis. So, now, the clipless hold my foot exactly where it should be, and my knee thanks me for it. Having used cages for so long, clipless is very intuitive.

While this has never happened, I've always been worried about hitting a bump or something, and one of my feet slipping sideways off the pedal, which might crash me. The clipless setup ( and even good cages ) prevent this. But you need to strap yourself in tight to the clips, and if you do that, getting out of them can be a lot more difficult than clipless. Honestly, the biggest problem I've had adjusting was getting my feet in.

Oh, yeah, and as atmdad said, they're great for Silly Commuter Racing. Even if they advance you on the food chain.

There's a bit of extra power always, but the strongest boost is more like "brake glass in case of emergency." Hills that used to be annoying are barely noticeable. ( Ass-Kick Hill is still going to take a lot out of you, though. ) Flats can be about the same as without them if you're trying to go at a slow, relaxed pace, but I'm finding that I get to and from work ( and other places I go ) more quickly with them.

My quick and dirty number is that I seem to go about 25 % faster at the same heart rate, now that I've got clipless. That's not based on an exhaustive survey, though, only on having an HRM and speedometer on my handlebars, and getting pretty used to how I ride.