I switched from HaloScan to Disqus this weekend (as a result of Haloscan's decision to stop offering their free service). The whole process was pretty painless due to this nifty tutorial and script. It took about an hour all said and done. I've been meaning to switch for a while since I didn't like the way HaloScan puts the comments in a pop-up window. I'm pretty happy with Disqus so far. Let me know if you run into any problems with commenting!