Positive Grid has some decent apps, their amp sims are actually pretty decent especially with the amp match features. When matching a DR 3ch with Marshall 1960 w 30s or a 5150II with the same cabinet, the A/B sounds very close, especially when used in a mix.
As far as amp sims, I have used for recording and for live aspects. One the best methods to utilize Positive Grid sims is to simply run the output into the amps FX return (bypassing the pre amp section) and you will be in tone heaven. For me, that is the best method of obtaining the usable tones for both heavy and clean guitar. When comparing to all the other amp sims, Amp One has a great Mesa sim tone, probably the best out of the bunch IMHO but only for recording due to the limited outputs.