Whether it's draining mana every turn, or just every time you cast it, makes no real difference. However, as a buff it is much easier to see where your mana is going.
There's also the fact that there is no provision for casting the same spell repeatedly. It's open spell book, cast spell, spell is cast, repeat until mouse hand is sore. 
The main reason why I find the original Celerity to be overpowered is because of the permanence of the effect. And mana is not that hard to come by. One AI opponent in my second-to-last game managed to get themselves 336 million of it. Admittedly, that's got to be a bug, but you can see there that there is the potential for the spell to be abused to the point where the game ceases to be fun.
Anyway, I freely admit that I may be way off in my assessment of how powerful it is, I haven't got that far in my test game to find out, and as with any mod it's the testing that really tells whether it is overpowered or not. I'm pretty sure that it could be, stacked up with the bonus from Tactician (for example) but I'm not sure if I will notice because I tend to maximise my unit initiative as a matter of course. It will also be interesting to see if the AI manages to make use of it.