I used to mix on a pair of sansui tower speakers; circa early 90's I think. Maybe earlier... I think they were 3-way, with 2-10" woofers, a midrange driver, and a tweeter. I was listening to some of my older stuff the other day on my blue sky prodesk, and it sounded pretty good.
I was also at a friends house the other day and listened to a mix of mine through his harmon and kardon setup. I had done the mix on some genelec 1032's. Surprisingly, the harmon and kardons sounded similar to me

. They did have a quite a bit of artificial low end though.
I'm convinced that as long as the speakers are full range, and you like the sound, you could effectively mix on anything you want. As long as you know how your system translates, you're golden.
So some thought for food: Say you have a decent monitoring system(as defined above), would it be best to stick to that monitoring system even if you can upgrade?
I'm thinking in terms of experience here...as far as i'm concerned, if you have tons of experience with a certain monitoring system, and its working out for you, then why spend an incredible amount of money to switch to something you have less experience with? I mean, look at ns-10's. Yep I've heard em...and no, I don't really like them... but a hell of a lot of people do.
Man, i really don't think the infinity speakers in my mini-van are all that accurate, but I bet I could do a pretty solid mix on them! I've have more listening hours on those things than anything else

!
just my 2 cents.