In the past I have had various effects units, beginning with stompboxes, then moving onto physical multi-effects (I have a broken M-Audio BlackBox and a Zoom floor unit) before going entirely screen-based. Having used the gear integrated into first Garageband and then Logic/StudioOne, I am quite comfortable with the notion of virtual devices, and the modelling in Ampkit is great, so it sounds very good.
Of course, I have no idea if a particular model sounds like a Fender Twin or a Peavy ValveKing, but I will say that all of the amps and cabinets have their own distinctive sounds and are almost all musical. The large array of pedals also allow you to shape your sound in almost any way. Presets give a good indication of what's possible, and are often a good starting point for customisation.
For me, beginning at the beginning was the best option, so I chose the amp I liked the sound of most and had the extra features I wanted (I went for the ValveKing) and then began adding a core quartet of pedals to cover EQ, compression/sustain, distortion and reverb. After playing around a bit, I removed the reverb and distortion (the amp does both quite well) and added a delay and a chorus.
These (and all the amp controls) are controlled via an Akai LPD8, which either goes on the floor or on top of the PA. The buttons and knobs makes it feel less computer-y and more real.
In all, I can't fault the sound - on its own or blended into a mix, I think it sounds great, and is far easier to carry around than a big amp. It would be great to see this available as a VST or AU plugin, because at the moment the recording process is quite convoluted, and I'll often switch back to using the built amp builder in Logic, and then just use Ampkit live. I would really love to be able to build my rig, control it in real time and process through my DAW. Hopefully that's next on the cards for the developer.
Before I got a bigger amp, I used this app for tones. It's okay. Don't bother buying any additional things though. None of them sound as good as the Valveking anyway. If you're gonna buy an amp, buy the Peavy 6534 on there. No other amp can get a tight, clear, and dynamic rhythm sound.