I used a pair of M1Mk2's for about a year. Thought they sounded real good, but a hair forward in the upper mids (which I like).
When we first got the desk setup in the control room we fired up the M1's and they sounded real good. No complaints at all.
When we A/B'd them against our JBL LSR-28P's, the Alesis sounded anemic in the bottom end and the JBL's sounded seriously lacking in upper mids. After a few days of listening influenced by the price tag differences we wanted to think the JBL's were more accurate and the Alesis had the problem. We actually fired up some measurement mics and made some pink noise plots and could see only a coupla dB difference in certain bands between the two. It became clear that the JBL's were indeed weak around the crossover point or the bottom end of the tweeter, and the Alesis were just the opposite (3-4 kHz). So we ended up dinking with the DIP switches on the JBL's to get them to cop the Alesis forward midrange vibe a little more and in the end, once they were tailored to sound the same, the JBL's were obviously more detailed and less distorted in the very high end. Basically the JBLs have that "very expensive tweeter" kind of sound while the Alesis have that hifi "fabric tweeter" sound ... a little softer in the very top end, less detailed, and more euphonic.
Anyhow ... that to say, I dig the M1's a lot and they helped us reveal a weakness in the JBL's and tune them. I can't really say the JBLs are worth 5X the price of the Alesis. I think we'd have been very happy mixing on M1s for a long time.