I have a -100 that replaced a dead -600. The guide speed is greatly improved, heat problems are gone and overall stability is better. My -100 hasn't locked up yet. It's a completely different board inside with a cooler running chipset.
Only downside I've had with my -100 is a somewhat weaker OTA tuner than the -600. I lost one fringe OTA channel that the -600 received no problem. Others over at AVSForum have reported just the opposite, so YMMV. And if all your OTA stations are strong, you probably won't have any reception problems. Overall, the -100 is a huge improvement over the -600.