I like some of the ideas behind Threshold-Limited Magery (aka Unlimited Mana). Particularly, I like that it lets mages always have a chance to do something (albeit with some risk). One of the things that annoy me about the standard FP magic system is that you can cast yourself to exhaustion and then there isn't anything else you can do.
But, using TLM has a several issues especially with Dungeon Fantasy type games. DF can be about frequent combats with a bit of rest between each. The standard magic system works great for this. Casters can blast, buff, heal, etc. till they are tapped out. Then can rest for a few minutes then are back at full strength to do it again. Sure power items and other secondary pools might not recover that quickly but the point remains.
With, TLM, things are quite a bit different. Players can run from combat to combat, without the need to rest and recover, and pull off big flashy effects. The down side is that after a several castings they are going to be nearer to the limit and nothing other than waiting days for your tally to wind down, which might not be an issue with long down time and travel times between runs. The way I see it, TLM makes dungeon runs easier earlier on, but as the players keep going, they will get to a point where the best option is to just leave.
At first, I was going to have both systems available for character to pick, but now I am concerning using a mixed system. The idea is to have the standard FP based system, where the players can use FP to power spells, but this switches over to TLM when they get to 1 FP. At that point they can keep casting, but all cost (or additional cost if they still had some FP to burn) goes to the Tally. This system has a Threshold of 0, so after burning down FP casters are risking backlash with just about any casting. FP (and energy reserves) still have their normal regeneration rates, and tally is reduced by 8 each morning.
So this system takes the most interesting part of TLM (risking backlash for the ability to keep casting), and replaces the least interesting part of the FP system (running out of energy and passing out).
I'll run this by my group and see if they want to give it a try. I'll post what I find if we do start using this.