So far as I tried it, there were no compatibility issues at all. The tricky part comes from the configuration setup. You'll have to play with the Audio and MIDI setup options in order to have it working properly, so my advise it'll be to check to several tutorials while you first use it, since the ASIO drivers could be a real pain in the neck, depending on the computer that you're using.
As regards the manual, if you're a guitarist used to just "plug and play", then you'll not understand a bit from it. If you have real technical knowledge, it's really helpful and very simple to read. Every single detail from the program is there.
Besides all that, the interface it's very user-friendly and very intuitive as well.
Once it's properly setup, it's AMAZING. It may have some latency due to the ASIO software drivers, but this can be solved by acquiring an external audio card. Works like a charm with my Lexicon Alpha
Stability problems? Not at all. No freezes, no crashes, no nothing! It's just like it's previous version, but with a few more options.
Performance it's outstanding. You can select from the several presets from the program and even create your own ones. Several amps, delays, reverbs, filters to choose from! From a Bon Scott AC/DC sound to the Richard Z. Kruspe Rammstein's sound (which is included as the Rammfire preset and it's great for metal playing).
I've been using it for a few years now and it became a vital part of my guitar compositions.
Sound quality? EXCELLENT. I used to have the GR3 and I find this one better in every single aspect. At first it may sound a little bit synthetized, but with a few adjustments works great. The more you use it, the more you learn from it, and the more you love it. It's a great product, and although it can be used as a stand-alone software, it's also recommended to get the Rig Kontrol foot pedal to get the best of it. Yes, it can be used with another MIDI foot pedal, but the Rig Kontrol was made for this program and viceversa, so that says it all.
As a Native Instruments user, I couldn't expect less from them. Great sound quality, everything adjustable and customizable to the user's needs and preferrences, very intuitive user interface and outstanding performance.
Worths every single cent and more, not to mention the free presets that can be downloaded straight from the Native Instrumens website.
Excellent option for both studio recording and live performance.
As a final thought, if you have the chance to get this software, don't hesitate. You won't regret it!