Are the MP66 (white trainer like ones) with spd plates any good with flat pedals? I normally ride spd's but want to ride flats a bit more, so would these do both or am I better off getting some of these for the spd's and some AM40's for the flats?

why not try DX's?

SPD shoes have stiff soles, which IMO are not great with flat pedals.

I've got some MP66's and whilst they're great as an SPD shoe for pretty much most of my riding, I wouldn't wanna use em on flat pedals cos they'd not grip anywhere near as well as my Vans do!

That's what I was thinking – probably get the AM40's or maybe some 5.10's for the flats.