As the guy who usually complains about the lack of cleanish and edge of breakup options in the Neural threads, well, you can guess my very first thought....
But I tried out the Eddy and the Mark yesterday. The (real) Peavey 5150 is probably my favorite high gain amp. Not sure exactly which on this Gain Stage one is based on, but guessing a later EVH one?
Anyway - definitely sounds decent. I like Neural's Nolly more, but for less than $30 this is for sure a very useable amp sim (for high gain tones). The CPU issue is definitely real though. It's higher than any other amp sim I have. I demoed it on an empty project, but I'd be concerned trying to use it on a project with lots of other stuff going on.
I also tried the Mark, which also sounds decent. The two distortion channels sound very usable and definitely don't suffer from any of the dreaded fizziness. The clean channel was less impressive to me, I found it had that weird 'emptiness' that I complain about in Nembrini and TH-U (and others) amps. The same with the Eddy, though to be fair the clean on the real amps are not great either.
One thing I did find was odd (on both) was that the 'master' seems to just be a linear output trim. There doesn't seem to be a way independently gain up the power amp vs the preamp. So that's a bit of a limitation. If you're just going for a good heavy lead or rhythm distortion sound, that may not be a big issue, but it is a limitation vs some other products.
So, basically - if you play metal or some variety of high gain guitar music, these are definitely useable and sound pretty good especially for the price. If your CPU can keep up.
Edit: for the record, I am running on a 2012 Macbook Pro. So not brand new, but it handles Neural, Mercuriall, etc just fine.
Edit
#2 : When I say these are decent for the price, I'm going on the intro price of 29. If they go up to 59 I'm thinking harder about it.