I just paid $125 to go from Editor 2 to 4. I get crashes if I leave melodyne on as a region effect when I save. I don't know if this happened on Editor 2, but otherwise I'm not getting crashes. I think I paid $150 to go from essentials to Editor, so to have the full blown Studio for $275 has left me feeling pretty happy. If you want to take advantage of this offer, I would say go for it. Even if you are concerned about some stability issues, you could take advantage of the offer and sit on it for a few months, whilst some issues are ironed out.
The sound design stuff is worth it for me. Especially more subtle use. Has so much potential for creative use and for helping mix sounds in. I've not used the timing stuff yet, but that's a great tool to have in the box.
My only reservation would be for those on Studio 2, in that you loose 'some' of the Audio to Midi functions such as pitch bend data. If you don't use this and don't envisage using it, then that's not a problem. (You still get Audio to MIDI note and velocity data).
Melodyne won't do anything further with the 2 series, but they will develope and fix anomalies, and any work cake do from now on, I can only imagine will be with Series 4 integration, though they may still have positive benefits for Series 2.
I'm so glad I took advantage of the window in upgrading from Essential to Editor, that the same opportunity to get Studio was a no brainier, especially as it was my birthday at the time