The puts and takes for me were varied, but consistently great moving to Milwaukee:
Drill - I had a brushed C3 and a smaller C3 I think was brushless. The M18 brushless is quieter, smaller and definitely more powerful. This is the smaller of the two C3's:
Impact Driver - I kinda hated the C3 impact driver. It was super-loud (esp. for a guy who already has tinnitus and hearing loss) and not gentle on the hand/arm at all. The Surge is obliterates it - really small, really powerful, really quiet and really smooth/gentle. On top of that, the modes make it the go-to tool for any kind of installation/removal - perfect application of power possible where the C3 was a loud beast (and mainly I used my C3 drill as a driver for anything that wasn't heavy-duty/decking). Huge fan of the M12 Surge. The Surge alone would make me choose Milwaukee again, and it is my most-used tool. (the C3 drill had that role before, hence why I had two)
Impact Wrench - the C3 impact was only OK. It worked well enough for lugs but struggled mightily with anything else. When I was doing control arms on my wife's van, it was of limited help on anything not a lug nut - it wasn't powerful enough for most things and it was too big to help with the rest (caliper bolts, etc). This did
not endear it to me. What's pictured below is the C3 with the massive 2767 (it was a similar size) and the M12 Stubby (which was more powerful - lol). I've since replaced the 2767 with the M18 Mid Gen2, which is significantly smaller than the C3 (not very much larger than the Stubby) and still multiples more powerful.

(my larger battery for the 2767 hadn't arrived yet - lol)

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I had a few other C3 things, but the story was the same for all of them. I don't miss any of them at all.
I think any newer platform is going to far exceed the C3 stuff. I see a lot of similarity in Ryobi's offerings these days. Beyond the stability of the battery platform with Milwaukee (I know RIGID and Ryobi also have excellent history on this point, but I don't trust any "house brands" any more and I find the Ryobi batteries bulky and their offerings a bit more entry-level overall than I want), what I really like about the current Milwaukee stuff vs. the others is the modes - it makes such a difference in the use of the Surge and the impact wrenches that make them far more effective to use. It's always a game of leap-frog - I don't think there are bad choices between Milwaukee/DeWalt/Makita/Bosch/Metabo in terms of platform - they all have their strengths. For me, I'm not ever going to trust a in-house brand that is subject to the whims of a Home Depot/Lowe's/Harbor Freight/etc. when it comes to a battery platform, so Craftsman/RIGID/Kobalt/FLEX/etc. I'm not actually sure where to put Skil/Porter Cable/Black+Decker any more, but they've lost all my trust too.