Im an Alesis MKII passive user, and theres not that much of a difference between the NS10s and these, i really wana get a set of NS10s, but i can mix on these and my mixes sound just fine on both, so theres no point in really getting the ns10s as of yet.
if you have the active versions they sound totally different to the passive versions of the alesis ones, the actives have a very coloured low end, and can be extremely boomy, the passive versions are not as bottomy, and have alot more focus on the mids (of which are very clear and accurate IMO)
i use studios with NS10s alot, and i prefer having a slightly different tone wth the alesis than just getting the yamahas as a sole reference monitor (until i can afford some £1k plus active monitoring)