I own a pair of 2900s and traded in a pair of CDJ900s to get them.
For me the 2900s are superior in pretty much every area except the display- the CDJ is better in this area simply because it displays an extra line of text (making track browsing easier) but otherwise the screens are much of a muchness.
The key improvements on the 2900 over the CDJ is the jog-wheel (much smoother, quieter, and resistance adjustable), and hot-cues (none on the CDJ). There are also things like the pitchbend buttons & master deck feature but I’ve never met anyone myself that have been big users of these things.
The switch to hard plastic for the play/cue buttons means that the main gripe with Denon players has been resolved.
I use mine linked to Engine software running on a small netbook- essentially to use purely as a big music crate screen. This is all controlled using the 2900s transport controls meaning I don’t need to touch the netbook.
I don’t have any experience using them as controllers, but I have also regularly used them as stand alone players playing from usb drives (with a link cable) and aside from the single text line on the screen they are fine for this providing your music drives/folders are well organised.
Overall I would say they are the best value for money media player out there at the moment. In my opinion it surpasses the CDJ900 (itself an excellent piece of kit) in every area except the extra text line on the screen of the Pioneer. In terms of build they feel every bit as professional as the CDJs and are in fact noticably heavier- still nowhere near the feel of the old CDJ-1000 but no modern media players are.
I’d also mention that for what it’s worth I also vastly prefer the Engine software to Rekordbox- I know this is likely personal preference but it just has a much more user-friendly layout and the browsing functionality when linked to the players is better.
If money was no object I would still take the CDJ 2000nexus / CDJ 2000mk1 ahead of them but nothing else. (Also worth remembering that even the 2000mk1 doesn’t have the excellent slip-mode but the lcd display makes up for this).
As somebody who has owned both, even if they were the same price I would take the 2900 over the CDJ900- but at £350 less per deck (here in the UK) it’s a no brainer and really highlights the Pioneer tax. And thats not a slight on the CDJs- they are extremely good, the 2900s really are just that good.
If there is anything else specific you’d like to know regarding either the 2900 or CDJ900 I’d be glad to help if I can! I’ve used a variety of other CD/media players (I use the Reloop RMP-3s regularly) but these are the ones I own / have owned.