Ok — Having played for just a few minutes, I'm gonna do this GamePro style:
GRAPHICS - 5.0
Beautiful. Henk Neiborg's work here is superlative and the animation detail is great. Feels like the classic Euro games of the 90s with an extra layer of smoothness. The boss creatures and environments are wonderful (so far) too. I'm going to go play Lionheart again just to look at more of his work; he's always been a genius with color and detail but the animation on display here is a step up from his 16-bit stuff. Catherine Menabde's cutscene graphics are very cool too and add a lot of spice and production quality to the intrastitials.
SOUND - 5.0

Superlative. This is probably the best overall sound package on any Mega Drive game ever released. Savaged Regime's soundtrack is brilliant and of wonderful quality, but what I wasn't prepared for was the overall cohesiveness of the sound and how
fucking good the sound effects are. It's no stretch to call the overall sound presentation arcade quality, and it really just makes you feel awesome while playing.
CONTROL - 3.5

This is where I have to be mildly critical—the game clearly wants to be played as a twin-stick style shooter and the Mega Drive's pad just isn't designed for these sorts of games; actual twinsticks or SNES-style pads are always preferable for these types of experiences, and I just can't get used to using the A,B,X,and Y buttons of a 6-button pad for Robotron style control, not to mention that doing so makes the roll and grenade buttons (both essential) harder to reach. The 3-button control style fares a bit better but doesn't work quite as well as something like Shock Troopers, which pulls off non-twinstick multidirectional shooting and rolling better. I would have also preferred for the melee to be activated by pressing attack close to an enemy as in Shock Troopers, vs Xeno Crisis's 'no input for melee' style which feels rather passive by comparison. These beefs are relatively minor but probably mean that most 'serious' play will happen on the newer platforms, which is a bit of a shame. If you have an SNES style input option for your MD it's probably less of an issue but I don't and I'm not really aware of any.
FUNFACTOR - 4.5

Said control beefs aside, my only other (really minor) quibble with the game in my super brief experience with it thus far is the popup/iframe window on some of the projectile based enemies that appear out of the ground is super short and sometimes leaves you feeling like you had no opportunity for evasion. That's probably just me sucking at the dodge mechanic without having learned the game yet, though.
That aside, Xeno Crisis is a blast to play. The visceral experience punches you in the gut from the word go, the shooting is tight and crisp, and the ammo and dogtag mechanics are great. Each room is pretty brief (at least in the initial levels), which keeps the pace brisk. You don't feel like you're up against endless Smash TV-style waves. You have a split second to plan out each room before the waves pour in, which gives you a brief amount of time to plan out your strategy with regards to environmental hazards, destructables, and barriers. Finally, the procedural layouts are a godsend and seem to stand a good chance of keeping the game interesting for quite a while.
CHALLENGE - ADVANCED

I've only played the default HARD difficulty so far, but this game is not messing around. You will lose very, very quickly if you're not paying attention, at least in the single player mode. The difficulty feels mostly fair other than that points mentioned above. I would personally like a slightly longer iframe window on hit to give the player a chance to regain their bearings and position (and maybe snag some ammo), but what's here works well enough.