I had a Speck Candyshell on my 3GS, and just got one for my iphone 5. Here are my findings:
Pros:
1. Great protection from drops. The rubber inside seems like it will absorb impact shock, and the lip will help for face drops.
2. Attractive imo. Really clean design and good construction. Fits well with the iphone design.
Cons:
1. Will scratch your phone over time. It's either the rubber or particles getting in between the rubber and the phone. The edges of my 3GS ended up scratched all over with the shell on. While the new case for the 5 seems to have a new rubber, I wouldn't trust this case for the following reason:
2. The fit for getting your phone in and out is TOO TIGHT. While this provides a great fit if you just leave the phone in there forever, I am sure that ingress and egress is damaging the buttons (power, volume, rocker). While it can take so much damage, one time is all it takes to ruin one. The tight fit also makes for scratches. I have put my 5 in twice, and removed it twice so far without problems. But I am not confident about the long run.
3. The speck case itself is highly prone to scratches. It's only been 2 days and the back as hairline scratches all over. If anything like my 3GS case, it will also scuff and dull over time. The bottom lip on my 3GS ripped down the middle as well.
overall I don't recommend the speck case. It's designed to protect your phone, but seems to introduce more damage. If you don't mind scratches, then go for it. I think it provides great drop protection. I dropped it once getting out of my car, and it slid on a gravel road. No damage whatsoever from the road.