Micros are fine for trading. Price-discovery is led by the big brother contract, and there are very few, if any, arb opportunities for retail. BUT price-prints do not always match the prints of the big brother. That you can count on! IOW, you may prefer MIT orders over limit orders, depending on your trade style or strategy. The volume overall is fine for MNQ, MYM, and MES... just not enough to create a price-print at every tick. Russell is thin for all it's contracts and imo, the micro M2K is only for swinging.
As for commissions, relatively speaking they are high. If you do your homework accounts can be easily opened allowing for ... 2 tick net breakeven for MNQ, MYM, and M2K, and 1 tick MES net breakeven.
I have monitored and done trades on all the micro-indexes. I prefer to trade the minis, but have nothing bad to say other than what's mentioned.
Trade On!