Aahh.. the good old mills debate ... i gotta say though.. although he appears not to be evolving at the moment, lets have a closer look:
1992/3 - waveform transmission 1 incl. Berlin, the subjects - dark matter/ step to enchantment.. Ruff as arseholes, hard as nails, raw, and at times gabba like proper hardcore techno
1994 - waveform transmission 3 - Still hard but with loads more disorientating synth work
1997 - Purpose maker which was so original at the time and pretty much singlehandedly gave birth to every other type of 'hard/bongo/samba/loopy' techno since
And then he's also got his axis style... more spacey deep dark and driving..
imo Since about 2000 he's been not that interesting but you can't deny him those first years of raw expression, quality tunage and blatent influence.. Hats off to the man... Yep shoddy mixing at times but thats obviously not his priority.. its not as if he watched the dvds/listened to liquid rooms and gave his approval for the whole world to see/hear whilst thinking 'yeah, my mixing was perfect'.. with mills it was always about the whirlwind of energy and frantic transitions from track to track... the overall bigger picture...
same as on lots of old dusty funk records... you can often hear the drummer miss a beat or the bass go out of time momentarily... all adds to the swing, the liveness, the energy.. and the MOMENT.. Anyway, thats my guess :)