I've used Overture, and the 5.1.0 release version has improved greatly over the betas.
The midi editing is very powerful, the ability to edit a piano roll or notation and have it be interactive is a real plus. You can also draw MIDI CC curves, and tempo maps, and they work very well to control any VI or VIPro parameter that you select.
Selection of VST instruments is easy, and the use of Vienna Ensemble and Ensemble Pro is also easy to do. Assignment of techniques to VSL patches is super easy as well. Just double click a technique, enter a keyswitch and off you go.
The one drawback is that there is currently no way (as Bill Reed says) to create expression maps. I think that feature is coming.
There's a free demo, can't hurt to try it out.