User Interface: Very simple "all-in-one" approach. The only confusion might be in the amp controls themselves which are exactly the same as the real gear. The microphone positioning is simple and effective. Drop down menus for speaker cabs etc. behave as one would expect. I would like to have numeric values for the speaker pan and output controls as they are a little fiddly.
Sound: I am a bassist and I use the 800RB live. I can set up this plug with identical settings to my rig and the sound is extremely close. If I add a little reverb and compression (more on that under "features") then "presto" I have my tone. GK's bass amp tone is very clean and doesn't add the color that an Ampeg, Fender, or other tube amps do...not everyone likes that, but if you appreciate the solid state sound of GK amps then this plug nails it. To my ears the speaker cabs sound good (but probably could be improved on) and the included mic models work well. All in all I believe that Audiffex has done GK bass amps very well indeed.
Features: Based on the review requirements this plug is not going to score very high in this area. Why? It does not include any effects, racks, or other bells and whistles. This plug emulates the amps, cabs, and mics and that is all. To me that is not a problem, but to some it may be. On my bass rig I add some compression and occasionally a little chorus. For the plug you might want to add some reverb too. I have a large number of dedicated plugs for those effects so I just add them to the signal chain and I have my tone. If you are looking for an all-in-one effects/amp sim then this is not for you.
Documentation: The included manual is complete if not exhaustive. I have no problems understanding it.
Presets: Audiffex included a large number of presets for all three amps simulated. I mostly tweak my settings and save my own presets but the included ones are a good starting point for the amps I am less familiar with (the MB150 and 2001RB). My only quibble would be that the some of the presets output levels need to be adjusted or they are too low.
Customer Support: Have not had to deal with customer support other than to order the plug and that was a joy (see value for the money).
Value for the money: I got this plug in Audiffex ridiculous 11/11/11 deal so, yeah it was a fantastic value. For full price it is still a good value if you don't need extra effects and you know and like the sound of GK bass amps.
Stability: Rock solid
Final Thoughts: I use and like GK Bass amps...I like this plug. I can use this plug on my DAW and I don't have to set up my amp or worry about volume issues with neighbors etc. and I can get a virtually identical tone much easier, that is the beauty of amp sims. However, if you don't have or don't want to use other compression, reverb, delay, or chorus effects then you should probably look elsewhere. GK Amplification Pro is a one-trick pony but it performs one heck of a great trick!