I figure many people in this topic area will be looking at MIDI controllers as they evolve. I just finished testing both the newer "Oxygen 49" and the "Axiom 49" MIDI controllers from M-Audio. I kept the Axiom after some deliberation, and considering the small price difference ($50 more for the Axiom). These two boards are similar but there are important differences:

(1) The Axiom has 20 memory locations to retain settings -- the Oxy has 10.
(2) Axiom has at least 4 key "zones" for layering and splits -- Oxy none.
(3) Axiom only has 8 rubber "drum pads" for more precise control of percussion and other effects.
(4) Axiom has an additional output ("espression pedal") and apparently a dual channel USB MIDI input to the computer -- Oxy has no extra pedal I/O and 1 computer electronic connection. Both can be powered in USB mode from the computer connect alone (but not through an unpowered hub). USB 1.1 specs.
(5) Axiom has "semi-weighted" keys -- Oxy the regular synth type. Both have aftertouch, however.

This last appears to be more important to some than others. I have a hard time seeing much difference -- this may change after breaking it in. The Axiom in its more substantial case (10 lb. versus 7.5) looks more "pro" and durable, though this may be an illusion (not the first time, if so). I believe most of the MIDI controller functions are almost identical. Both series come in 25, 49 and 61 key versions. The smallest lose a few controller knobs, etc. and perhaps something in the I/O group.

After really trying to use this new KB for a while, I'll comment on its actual utility. We may here from others who are also picking up these units.

An edit here to clarify:
Only the Axiom line KB has aftertouch potential. The Oxy line does not. Both have multiple KB velocity settings, however, which is what I was thinking of when I made this comparison.

I have noticed that aftertouch of any kind is hard to find in the under-$500 USD controller boards I have run across that have other features I want. For example, the Novation LE line (nice KB, but synth action, no aftertouch).

I have been looking at the specs for the Axiom 61. There are no dealers in town. I know it is very subjective, but I was wondering how stiff the "semi-weighted" keys are compared to a synth. My wife has a Clavinova, and it is rather stiff.

Bernie

Bernie, I would not rate the Axiom keys as "stiff". In fact, I had a difficult time seeing much of a difference in resistance between the so-called semi-weighted keys and the regular "synth" keys on the Oxy 49 and others. I would say the feel is no more stiff than my Yamaha 9000 or other such models. However the "bounce" is moderated, and I find the overall effect rather good. Not at all like a true weighted KB in any sense. But I cannot play as fast as I can on my Yamaha arranger boards. Not yet, anyway.

I actually like this model quite a bit and find the $50 USD differential worth it for the additional functions. It is -- however -- noticeably heavier than the Oxygen model. Yet still far lighter than most arrangers and many full synth boards.

If Fran Carango sees this thread, perhaps he might chime in with his impressions, since I recall he was buying an Axiom 49 recently. And he would be a far more active and critical user than I am.

Here is a follow-up on the M-Audio AXIOM (only) controller -- I decided to take that one as opposed to the "Oxygen" model. I purchased the 49-key model (25 and 61 key available):

-- I like the keyboard feel, after all. It is somewhere between "synth" and "piano" feel, not as wonderful as a good hammer-weighted key set, but perfectly acceptable. And for the price, I think it's an achievement. The board iteself is solid and give a better impression than many of the ultra-lite plasticky boards available. Before finally deciding to keep the Axiom I also tried out a comparable Novation controller --- though it has some nice features, I found the keys a little more "synth" in feel, though others reviewing it disagree, praising the "semi-weighted" feature. I also again tried the CME UF series of boards; I like the look, feel and apparent quality, but the lack of any memory to save custom settings threw me off. The Axiom does have space for I believe 20 custom presets or setting groups.

-- The controllers (rotary and sliders and buttons) have all worked so far. I think the feel is acceptable. Many competing boards are really cheap to the point of making the Axiom and a few others stand out.

-- There are apparently various problems synching with various software packages. The "presets" may not be up-to-date for certain software revisions. I base this partially on bulletin board comments from others, as I have limited software and short experience with these. The "Enigma" software that comes from MA with the controller board appears fully-featured (it is basically for setting up the board in many variations and adding and modding presets). However it is less than easy to use, and I have had a number of problems trying to set up my board.

-- On the other hand, without getting into the full use of controller commands, I have successfully used it with OMB, OMB-Forte, MA Key Rig, and a few other things. I have not yet mastered the "drum pads" on the board, though they certainly seem useful down the road.

More feedback coming if I learn anything important.

I now have my Axiom 61 and love it. The feature/price comparison is a no brainer. I agree that there is a learning curve, but, with that many features, it is no surprise. The Enigma forum is real helpful. http://launch.groups.yahoo.com/group/M-Audio_Enigma/

I recently tried several controllers and went home with the Axiom 49. The superior feel of the keyboard sold me. I have been using it successfully with Finale Allegro. The aftertouch I thought would be useful for pairing up with my T2, if I want to split a solo voice to another keyboard. The pads seamed like a nice touch as well.

I saw a good set of video reviews on Sonic State about 25 key controllers including the Axiom 25 http://www.sonicstate.com/articles/article.cfm?id=100