It's surprisingly accurate, not 100% all of the time, but so close that you will not be able to tell the difference in a mix or playing live i.e. anywhere that counts.
Accuracy is determined by several factors though such as how long you spend refining the profile, and to an extent the guitar used when refining can have an impact.
It also has to be said that it will capture the sound accurately within a range, so if you profile a high gain amp and then dial down the gain on the Kemper it will continue to sound like your original amp, but the further you dial the gain beyond about a quarter of the throw it will start to deviate from the way your actual amp responds when adjusting gain, it will however still sound like your amp and you can get sounds that your own amp could not achieve this way. The dials offer rather more extreme ranges than any real world amp by at least a factor of two in my tests, so while a quarter of a turn sounds like it may be nothing it's actually quite a difference.
In terms of practicality in your situation, it would probably be great. Keep in mind it profiles your signal chain so crap in crap out, good stuff in, good stuff out. It's also very convenient to have just to store a sound of a session and then have it for future use for either re-amping or continuing hte project in another space.
Overall I think it's a revolutionary product, it's a great complimentary tool to have if you've got a nice amp collection which can save you a lot of time and effort if you tend to use the same tones from your amps, and it's the best amp sim out there if you have no amps of your own or if you like to explore possibilities in a safe environment and go beyond what the original amps can do without cracking out the soldering iron.
Oh and it helps that it both sounds and feels like playing the original amps in the control room.