Why don't you recommend this? Metal Slug is awesome. Yes, yes it is. This port of it however, is not.
What you get is a minimum effort emulation of one of the greatest arcade games of all time. Wether or not you enjoy this release depends on how much you care for flawless emulation - as someone who owns a MVS with a couple of Metal Slugs and tried some other alternatives to get NeoGeo running on something that is not a NeoGeo I don't think Dot Emu did a good job so far. In it's current state it is definitely not for the Metal Slug-aficionado.
The emulation doesn't provide enough options to tailor the visual output on screen to your liking. You get a set of resolution options but none of them allow the display of the game in a letterboxed format - resulting in every other row of pixels getting stretched. Which looks just terrible, especially considering that Metal Slug 3 is a pixelart masterpiece and the lack of not caring from the Developer for preserving it's beauty for newer generations of players pains me greatly.
There are ways to force the game into "correct" resolutions depending on your screen-size, but you have to fiddle with gamefiles and edit these in order to fix something that should be there from the start. Same goes for the "visual enhancements" - which in their current state are pretty much crap, a super-blur and a scanlines effect that doesn't adjust to your resolution. Again, somebody from the fanbase fixed that - with a bunch of files you can replace, yet again these options should have been there from the start or somebody should have put some thought into implenting them. Also, for whatever reason running the game fullscreen results in a lot of weird issues and glitches - one of my mates controllers doesn't properly respond in fullscreen and we are both having some serious stutter and framerate issues (issues that aren't on the original hardware, mind you - we know the game has some serious slowdown at times) - it's like it is running out of sync or skipping a beat and then suddenly gets it right again. Switching to windowed mode fixes a lot of these issues, but seeing my desktop in the background just feels wrong on so many levels. Also, the forums are full of people having issues with mini-freezes and inconsistent framerates - which is all the more weird since NeoGeo emulation has been around for a while now and stuff like that shouldn't happen nowadays, not when mobile phones are easily more powerful than the actual arcade cabinet this game originally ran on.
Something else that bothers me is the constantly desyncing music that plays too fast at times and sounds weird in the process. The same applies to some soundeffects (most notably the GO! and Continue-counting noises are just wrong). From what I read this has been known since the beta - and again nobody cared.
Finally, the one sole reason to get this, online-coop, is another minimum-effort job. You get to play people from your friendlist - that's it. No lobbies, no matchmaking, no indication how good or bad the connection is. They didn't even bother to code a proper "invite" screen, basically it is a Steam pop-up that you have to control with a mouse (which might be different in Big Picture mode, but I didn't try that out). Online is better than Kaillera stuff - but it introduces a lot of problems that could make this game unplayable if you are used to the offline-experience - rubberbanding, massive slowdowns (again, not the kind the original machine has) and frame skipping being the biggest ones we encountered. It kinda gets the job done, and is probably better than no online-coop - but it is not a convincing effort.
And for all you arcade-operators out there, you have NO control over the dipswitch settings. A couple of options are available under settings, but you cannot change the number of lives per credit or the number of continues you start with, and just you forget about freeplay right away - all of these settings are "hardcoded" into the bios-file that comes with the game ROMs. Also missing is the attract mode / opening of the game - which is a damn shame and no NeoGeo bootup screen either.
You simply might not care for all of this, the game itself kind of works anyway and that might be enough.
Emulation-issues aside the game is great on its own, but not my favourite in the series (I personally favour X and the original Metal Slug over Metal Slug 3). I am not too fond of the shmup-segment towards the end and feel that especially the final mission could have been split into two parts. It is just not as "on point" as the prequels were. Also, while the spritework is fantastic, I feel that the backgrounds in 3 are less detailed and exciting than in X or the original. I particularly miss the spectacular destruction of big parts of the level, like the residential blocks in the occupied town of the original. They kind of overdid it with the various Slugs as well, I liked it better when there were less "Super Vehicles 00X" - however, all of this is highly subjective and there is really no alternative on Steam to the greatness that is original, classic, japanese arcade run & gunning (if you are thinking of the ♥♥♥♥-poor Intrusion 2 right now, you are wrong!). Which is all the more reason I will not recommend this lackluster port in it's current state - but will happily change that once a few updates fixed at least the emulation issues.
Metal Slug deserves more care and a lot more love from the devs!