A great, albeit shallow, shooter.
HL2DM is essentially a modification for HL2 that was released on Steam as a full-fledged game for a whopping $5. This game is actually quite good for the price, but it isn't worth much more than $5 or $10.
For one thing, this game is somewhat shallow. There aren't any unlocks, there is absolutely no ranking system whatsoever, there are no achievements, there aren't a ton of maps, and quite honestly the multiplayer in this game feels like a tech demo -- literally HL2 thrown into a deathmatch scene. Valve didn't do much, if anything, to address the fact that this is a multiplayer shooter, not a singleplayer sci-fi epic.
And yet... this game seems to grow on you, over time.
One of the great things about this game is its simplicity. You can hop right in and not feel too discouraged or daunted, save by the few amazing players out there. There aren't a whole lot of maps, but the ones that are present are quite well designed and offer some intense firefights. Although the weapon selection is meager at best (we're talking no more than 8 or 9 weapons here, folks), they all service quite well, and are fun to use (especially the gravity gun and crossbow). The graphics are well done with the Source engine, and provide a fun and immersive experience. The sound quality is also great.
If anything, this game is simply too shallow. Although it is well done and is definitely worth $5, I'm not sure I could say that it is worth much more than that. It feels like a tech demo -- if a fantastic tech demo. If you enjoy multiplayer shooters by all means, pick this up... I did, and I'm certainly not disappointed. If, however, you only like super polished, top-of-the-line and insanely balanced shooters such as Counter-Strike, Day of Defeat Source, Team Fortress 2 and Halo _____, you might want to give this game a pass. That is, if you can't stand the fact that this game isn't as big or "OMG I WANT TO UNLOCK THIS THING OMGOMGOMG"-y as other titles.
In any case, this title is awesome, and I say go for it if you enjoy multiplayer shooters. Well worth the price of a Starbucks coffee.