I bought Slayer2 right when it came out. I had played aroung with Slayer1 before, so I was a bit disappointed by Slayer2 as it missed the "bite" of Slayer1. This has been fixed with the 2.5 release of Slayer2 meanwhile.
The GUI is really great, if the readability of some controls would be improved, it would be perfect IMO.
The sound is simply outstanding! There is no other other electric guitar simulation that I'm aware of that comes any close to this quality. Brilliant!
Regarding the abilities to change the sound character, there's nearly every feature I can imagine. The only weak point here is the "humanize" feature, because it doesn't offer as much playing alternation as I would expect from such a function.
The documentation is quite good. All features are explained widely here. I only miss some playing hints here since it's not very easy to "think" like a guitar player if you can't play guitar (like me). Some tutorial would be appreciated very much.
The presets could be tweaked a bit better. They are all quite usable, would I always have to tweak some settings before they fit into my mixes. Also, some of them sound a bit too identical IMO.
The customer support is simply great! I had some trouble with Sonar's VST wrapper when I got Slayer2. After some email exchange with the devs they gave me a bugfixed beta-build that solved my issue.
For the VFM: Just compare it yourself! Download the demos from reFX and test them. Then compare the price/features ratio with what you get for this money from other companies. Slayer2 is definitively worth one's salt.
Slayer2 is solid as a rock. I wasn't able to crash it here, even if I overloaded my system with plugins while running low latency.