Okay, so I volunteered to be one of the guinea pigs for the Neo Geo Station, and I had a little time to play Metal Slug. So here's my thoughts:
Overall, the emulation quality is very good. It looks like the ports were done by M2, and it shows. The slowdown seems to be in all the right places, there's no input lag, and the game looks razor sharp. I did notice a graphical glitch with the second stage boss, which has a weird, black hitflash instead of an orange one. It's a minor issue, but seriously... what the fuck? This should be perfect, especially for $9.
Audio quality seems to be fine, though I did notice an occasional clicking sound when I was fighting the first stage boss when shit was really getting chaotic on-screen... can't remember if that was in the original off-hand, but it probably was and my memory's just failing me. Again, a minor issue if it is actually a discrepancy.
I tried the online mode briefly. Lagfest. Don't bother.
The frontend itself is really, really nice, though. I like how everything is really well organized and pleasant to look at, and there's no shortage of visual options. Thankfully, you can turn all the stupid smoothing filters off and keep the proper aspect ratio so that it looks absolutely mint. Hell, I think this may actually be sharper than any way you can actually play the original cart, or even better than MAME/Nebula/Kawaks/Final Burn/NeoRageX. It just scales really, really nicely.
One issue I did have, though, is that this is the AES version of the game, and it seems to limit you to 4 credits. I didn't see an option to switch to MVS mode, or an option to get more credits. And if that's the case... that's fucking stupid. Now, I'm a real man and I play my arcade games on one credit, but it's downright retarded if they don't at least give you a freeplay option of some sort. Maybe I just missed it in the menus or maybe it's unlockable or something, but if it's not there, that is a problem.
So overall, is it worth replacing your Neo cart with? Well... maybe. There's some issues, but they're mostly minor, and I really don't think the game has ever looked as good as it does here. It's truly remarkable how sharp it is. I'm still on the fence with this issue myself, but despite the minor emulation issues, this does seem like a very strong effort.
If you're not particularly anal about the emulation, then yeah, you should buy it. This is the best port of the game by a fucking mile (the only competent one, honestly), and it's probably better than whatever you're playing right now.
I might pick up Samurai Shodown later to see how that measures up.