Owning most of them and having tried pretty much everything out there - because I
had to rely on amp sims for almost 2 years, my opinion is that they're pretty much all hit and miss.
And the "miss"
by far exceeds the "hits". Which is why I own so many of them and have used them in almost every possible configuration.
But sometimes, out of nowhere, one will just work for something. I have a song in the works for which I used Waves GTR because I suspected it could get me in the ballpark quickly for the kind of tone I was after. It's the first thing I tried and it just worked beautifully. I don't think anything else would sound half as good.
Same for the Hiwatt model in Amplitube - I have one slow blues kind of song which I revisited last night for which that particular model is absolutely perfect. Everything else I've tried failed. Usually, I bypass the cab in AT and use impulses, but that song is the one case in a million where AT worked as is for me. Using impulses actually took something away, so I reverted to the cab emulation.
Honorable mention to S-Gear, which gets the closest, imho.
One thing I can tell you is that I have learned an awful lot by using amp sims, and one of the most important things I've learned and on which I'm getting a grasp is not to trust my ears as a guitar player. I need to take a step back and put on the engineer hat.
Most of those amp sims can be a whole lot of fun to play - and there are at least a few very cool models and effects in each of them. But when you've spent so much time trying to mix them, you start to recognize these little flaws hidden in those amp sims, little things that can drive you nuts when you're trying to mix, because, oddly enough, that's where they become more obvious. And no amount of EQ'ing will work w/o severely compromising the tone.
It's a bit like those abstract pictures in which there's a hidden image - once you've tuned your eyes to see it, the hidden image is crystal clear.
So my verdict on amp sims is that they don't necessarily make your life easier. They can require an awful lot of work and tweaking, same if not worst than trying to learn to properly mic a cab. But sometimes, they just work.