I use LightZone. When I first tried it back in December, it wasn't very stable on my machine (Win XP, 1 gig of ram), but I liked it. I bought it at V2.2 at the end of February and it has been stable - no disappearing from the radar screen so to speak. As for speed of conversion, it is probably not the quickest out there. For a 5D raw file needing minimal adjustment with a single ToneMapper, I can open, adjust, export a TIF, and save the LZT file in around 3 minutes. 5D file doesn't need much sharpening or noise reduction - if I added those tools to the stack, that time would go up.
For high volume work, LZ needs ram. But for me with photography being a hobby, the speed of conversion issue isn't that big of a deal. One of these days, I will get another gig of ram. I have RSE and DPP and can get a better result with LZ. LZ has a unique tool set that I see as complementary to "traditional" converters. I did a middle school play last week with ~400 pictures. I only used LZ on the cast portraits. For the high volume stuff of the play, I reverted to RSE to process. LZ is a good quality converter, but speed is not it's strong suit unless you have ram.