I have a protool 18V drill for about 2 years now; allthough the batteries look different (they are slightly bulkier and have some protective rubber around the bottom edge) the contacts are compatible with the festool drills.
When I bought the protool duradrive drywall collated screwdriver I tried the 12V batteries in the 18V drill and they worked BUT at a much lower rpm. (I also tried to slide an 18V battery on the 10.8V machine, but (as to be expected) it wouldn't fit.
So the batteries of the new C12 will work with the T15 but it will make it run at a lower speed.
Sidenote: There's a thread on a German powertoolforum about a guy that was complaining that his T15 drill didn't reach the rated max rpm, and the conclusion was that those numbers were only reached with the NiMH 15.6 batteries (which are also compatible with the T15, but not with the Li Ion batteries that come with the drill)
A little over a month ago I had a problem with my protool drill which required a warranty repair. Allthough I can choose from a small arsenal of backup drills, I really missed the centrotec and the angled chuck. (I do also have a bosch I-driver, but I really don't like it). When the new C drills were announced I decided I would check them out as soon as they would be available.
Allthough all of the dealers I frequently visit had the carvex and the small rotex in stock soon after their release, only one could show a few of the new C-drills. I decided to get the C12, because it was significantly lighter than the protool drill, and, if the 1.3Ah would turn out to be insufficient I could also use the 2.6 Ah protool batteries in it.
Sofar I really like it, the 1.3Ah batteries are a lot lighter, and I don't mind putting them on the charger every once in a while (the charging also takes less time). I like the belt-clip, it is shorter than the clips on other brand drills, with the tip a bit angled out, I actually put it on my belt, while I hook the other drills to a pocket. (By belt I mean the belt that holds my pants up, I don't wear a toolbelt).
The led light works fine too, it's a lot brighter than the ledlight of the makita drills and it's placed at an angle. It sometimes casts weird shadows, depending on which chuck/bit is used, but the tip is well enough illuminated. (It doesn't stay on after you let go of the trigger, but the trigger has a short free-stroke so you can use the light without turning the drill)
Before I ordered it (they didn't have the set-version of it in stock) I checked with someone who recently got the C15 if it had the R?hm type keyless chuck, which it has. (The chuck on the C12 is actually a bit heavier than the one on my protool drill.)
The right-angle chuck is lighter than the protool version, and easier to adjust. The excentre chuck has come in very handy.