How stable/usable is Quassel IRC?

Well, depends on what you ask from your chat client. First of all, all the devs use Quassel IRC as their main clients and have been doing that for a while. We all share one Quassel Core, so that component is getting quite a stress testing...

Quassel Core has been rock stable for us for a while, and we have ironed out most Client crashes recently. So yes, Quassel IRC is certainly stable enough to be usable, at least on our systems. If it isn't for you, please report bugs, so we can fix it :)

Now for features - as I said, that depends on what you need. Quassel does not have all the nice features yet that shiny clients like Konversation give you. On the other hand, it certainly already now is much more convenient than text-based clients like Irssi. There also is no plugin support yet, so if you rely on your /slap script, you'd have to wait...

If you can work with issueing basic IRC commands via the input line, rather than relying on everything being clickable, you should be all set to start using Quassel!