I am a member of the "highly enthusiastic" AN kit forum, but despite my bias I will aim to answer your question as honestly as I can! For five years I have lived with and listened to the interstage monos and also my Kit1, a 300b stereo amp from Audio Note. In that time I have also had a lot of fun restoring several vintage amps and so I've had the opportunity to compare them all.
In the end--and to try to answer you question--the monos are terrific amps. I find that most people who are into DIY are as into the process as they are into the result. Some days you can count me in that camp, and on others, well I'd rather just be done and not tinker. For those who will endlessly change components I believe the monos are an awesome test-bed. The configuration is sound and the quality is excellent. Andy Grove, AN designer, penned the circuit and so you know it's got potential.
However, if you are just looking to build the amps and be done with it, I think you will always have some thought about upgrades and that doubt could linger. That's where I am currently: trying to chase down the last bit of resolution and performance before spending obscene money. Who knows, maybe you will like what you heard at first listen and never have the urge. Off the carousel as they say! Ahhh, wouldn't that be nice. Well, this is (like the Kit1) an off-the-carousel type product. Or at least it's a lifetime keeper.
To me, the amps are terrific sounding. Better bass than what many will tell you is possible from an 8 watt amp. On my ANE I greatly prefer the sound without any sub and it fills my large room with truly moving bass. Midrange is astounding. My other SET amps are pretty good there too but the mono is the best I've had at home. Overall, it's for sure worth the cost of admission.
World class? Certainly. The best ever? No, probably not. There are level 5 amps that have much more dear transformers that I just "know" would be better than what I have in mine now. I have a friend with level 5 amps, and another with level 4. While I have not compared them, they all share a similar house sound. If you put together the monos knowing that sound and wanting it, I believe you will be happy.