I have been using my Festool CSX for several years now, and it is my go-to drill for doing cabinet and installation work. The only two real issues I have had with it are the fact that is uses the Centrotec bits and the battery it uses doesn't fit any other tools.
Milwaukee recently released their version of the CSX using the M12 battery platform. Mine got delivered last night and I thought I'd do a quick comparison.
Both drill/drivers have quick detach heads, a magnet to store bits, and a LED to light up your target area. Both have two speeds, an adjustable clutch setting, and allow you to use standard 1/4" bit directly in the drill without attaching a head. Both have approximately the same weight, balance well in your hand, and will stand upright on a flat surface.
The CSX comes with 3 heads: a standard 3/8" chuck, a centrotec bit holder, and a 90° bit holder.
Milwaukee Installation Driver 2505
The 2505 comes with 4 heads: a standard 3/8" chuck, a centrotec bit holder, a 90° bit holder, and an offset bit holder.
The two drills are very similar in size and shape
The Milwaukee has less depth, and is about the same thickness
The Festool is a bit shorter. You can just barely see the Milwaukee underneath it.
Initial thoughts: The Milwaukee has noticeably more power. I was able to drive a 1/4" x 3" SPAX lag screw all the way into a 4x4, and I could feel the torque pulling it out of my grip. The Festool was only able to drive that same lag about half way in. I tested with 2ah batteries in each. Switching between the various heads was equally easy with both drills. The Milwaukee has the forward/reverse switch as an electronic button on top instead of the traditional mechanical switch near the trigger. I don't hate this (yet), but it will take some getting used to. The Milwaukee takes standard 1/4" quick-change bits. This is a huge plus for me. The Centrotec bits are nice, but they are expensive, hard to source locally, and not available in as many configurations. The bits slip into the Milwaukee chuck easily, and are quick to remove.
I plan to use the new Milwaukee driver heavily over the next few weekends, and I'll update with any new impressions.