Until recently Iíve been sharpening my turning tools on an old Record Power belt and disc grinder. Itís worked really well.
I also sharpen some of my metal working tools the same way. I only really bought the new machine as there are a better range of different belts commonly available for it.

If I did a lot of flat work (carpentry) then Iíd maybe look at a wet stone grinder. I had one before but it was too slow for turning tools. Sooner of later you want to reprofile a tool and 90 RPM doesnít cut it - pun intended!