I have been running Elements of Magic in my home campaign for over a year now. My players really like it and I find it much easier to DM than the core magic system. (Frankly, trying to remember all the fiddly bits of different spells drove me crazy.) The flexibility of the magic system is awesome. The possibilities are endless. Frankly, it's the best thing since Arcana Evolved, once you do some minor tweaking.
However, I feel that the power level of EoM is not quite the same as core spellcasters. I can't say that it's higher or lower, because really it varies. There are some low level spells in D&D that are harder in EoM and some the other way around. I would not recommend letting your players mix the two magic systems in your game. (Not to mention that you'll have to know two different magic systems.)
When it comes to pure, raw damage, your players may be a little disappointed. A 5th level wizard can cast a 5d6 fireball for 20' radius and at like 500'. A spellcaster with 5 caster levels in EoM has more flexibility in their spell choices from Evoke Fire, but to get a 20' radius at 500' would mean they're only doing a couple of d6 of damage. (I just can't remember the MP cost of long range right this moment.) Given some time to get used to the wonders of what you can create with Spell Lists under EoM, they should come around. I know my old-timers in my game did.
I do have some complaints about EoM. There were some bits in the spells and some examples given that were too munchkin for my tastes. I upped the cost of a few things simply because they were too open to abuse. Even with all that, if you have smart players like I do, you have to watch for odd and dangerous combinations. I highly recommend a thorough read-through with some consideration of editing to suit your game. (Partially real illusions, for one, are open for abuse. Summoning is a little odd because you have to pay twice as much MP is you want a creature to actually follow your orders. I rewrote big chunks of both sections.)
If you have ever played a point-based system like HERO, then you understand that any point-based system can be abused by someone who really tries to squeeze the points. Some combinations, while technically possible, just should be allowed. Fortunately, my players are fine with the argument that, "If you don't use that, I won't have my NPCs use it; okay?"
But from me, the final word of Elements of Magic is that having run it for a year, I am never, ever going back to the 3.5 core magic system. No way.
