I have the C12 plus chuck kit plus depth-stop chuck. I bought mine and have no affiliation with Festool other than as a customer. (I do however post on the Festool Owners Group a lot.)
I think it's a great drill - very high quality and powerful for it size and voltage. This is probably because of the brushless motor.
With the chuck kit, it is very flexible. With all these chucks, you can get into places not possible with other drills.
Although it takes some getting used to, I actually like the design. When you slide your hand up the grip and pull the trigger with your middle finger, it puts your arm inline with the chuck. This allows you press directly in line with the chuck and not torqued as you do with most drills.
Another nice feature is that both the drill AND the charger have overheating alarms. If you overuse the drill and the drill and/or battery become very hot, the drill will beep at you and wait until it cools down. If you stick a very hot battery in the charger, the charger will wait until the battery has cooled down before charging it. I know this because it occurred to me multiple times. These features saved the drill and batteries several times after driving 65-70 3" X 5/16" lag screws into tough lumber as fast as possible. Even after doing this many times, the drill is still in great shape with no damage.
The C12 has two downsides that I can think of. First, it doesn't use LiIon batteries like my Makita impact driver (I'll get to that in a minute). The 2.4 Milliamp, NiCd batteries on my drill have performed for two years and are still going strong. But they do take longer to charge and are heavier than LiIon batteries.
The second downside is cost. They are pricey buggers. Whether the higher quality and flexibility is worth it to you is entirely personal. It was to me.
I also have a new, black and white Makita impact driver. It's a gem. It drives huge screws without transmitting torque to your wrist. I find that amazing. Also, it is very light and the batteries charge in 15 minutes flat.
However, it too has a couple of downsides. First, while it functions well and is a joy to use, the quality and flexibility is no where near as high as Festool. It's still very good , but a notch below Festool. Second, I'm having some problems with either the batteries or the charger. When I leave a battery in the charger for more than a couple of weeks, it's dead when I try to use it. If I take it out and reinstall the battery into the charger, it starts charging and seems fine in 15 minutes. It works, but it's a bit disappointing.
Regards,
Dan.