I've got the Korg MS20 plugin and it's ace! Stable tuning and the ability to sequence and store patches. I think it was only about $40 too. Time was you had to buy the 'Legacy Collection' (Mono/Poly, Poly 6, MS20, Wavestation) that came with a USB MS20 controller, but they've since made the plugins available individually.
Toying with the Poly6 just for the arpeggiator, but I should be less lazy and learn to program the MS20 [i]properly[/i]...