I know most people haven't played much if any SG-1000 games, but I've now played about 90% of the small library (about 60 games), so I thought I'd give my thoughts to anyone who might in the future want to play some games on Sega's first gaming system.
The games aren't the easiest to track down since they're all Japanese, but they're very easily emulated, since any emulator that can play SMS games can play SG-1000 games.
I'd put the SG-1000 on par, technically, with the Atari 5200. They came out around the same time, and graphically, some of the games are similar. So it's mostly arcadey titles. You know what I mean.
Good SG-1000 games, in no particular order:
-Girl's Garden
This was the first SG-1000 I played, and it's still very fun. It's a very charming arcade-like title, and it also happens to be the first game ever designed by Sonic Team's founder, Yuji Naka. You basically play a girl who collects flowers for her boyfriend in a garden. In each round, you have to collect 10 flowers, while avoiding obstacles and bears. Difficulty scales up quickly, but I've gotten to about level 10 or so. I also wrote a short FAQ for this game over on GameFAQs a while ago.
-The Castle
If I said this was the prequel to Castlequest for the NES, some people I think would be turned off by it. Because as I understand it, Castlequest has a lot of very negative hype around it. But, for an SG-1000 game, it has great graphics. It is the most expansive game for the system, with a lot of exploration and platforming. Maybe not for everyone, but I think it's quite fun.
-Elevator Action
You know what Elevator Action is. I think everyone has played this game, and the SG-1000 port is pretty good. Maybe slightly worse graphically than the NES version, but just as fun.
-Gulkave
One of several shmups for the system. This game is graphically the most impressive thing on the system by far. You might even say it's downright beautiful for games of its day. It even has paralax scrolling. It also is more fast-paced than other shmups for the system, and it has plenty of stuff on the screen to shoot. It's a horizontal shmup. The one downside is that I find it REALLY hard.
-Exerion
There is a version of this vertical shmup for the Famicom, and remarkably, this version for the more outdated system may be superior graphically. Although, it's nowhere near as good looking as Gulkave. It's a vertical shmup, and while it's not as furious as Gulkave or good-looking, it's not as unfairly hard as Gulkave either. So take your pick on best shmup.
-Drol
I don't think there's a point to this game other than shooting bad guys, but maybe I just missed the great objective. It's still fun though, because it's one of the few action games for the system that isn't hard, and the little robot guy you control is cute. The controls are very responsible, too.
-Flicky
If you've played Sonic 3D Blast, you probably remember what Flickies are. They're little birdies. This is a very simple game where you play an adult Flicky, and you hop around platforms to collect baby flickies and bring them back to home base. You have to avoid cats, and it's a cute, mildly addictive title. I only had two problems with it. First, the backgrounds are drab looking, and secondly, I kept hitting my head on the ceiling when I tried to jump to a higher platform.
Honorable mentions (also decent):
Doki Doki Penguin Land
Golgo 13
Star Jacker
Hustle Chumy
Bomb Jack
Dragon Wang (not a typo)
Worst games. Don't touch with a 10-foot pole:
-Ninja Princess
I REALLY wanted to like this game. It's basically a "Pocky and Rocky" style shooter, and I really like Pocky and Rocky. Plus, it had a female hero, and we needed more of them in the 1980s. Unfortunately, your character moves SO SLOWLY, that it's impossible to dodge anything. This game is borderline completely unplayable. And speaking of borderline...
-Borderline
This is a very crappy game where you drive a slow car and shoot things. Don't let that make you think it's like Spy Hunter. It's not. The controls are sluggish, the game is ugly, and it's not fun.
-All the sports games
The sports games aren't very good. Some of them suck more than others. Champion Tennis is probably the worst one. And Champion Baseball makes "Baseball" for the Gameboy look like MLB 06 The Show.
-N-Sub
Probably short for Nuclear Sub. It's not a hard game, but this game plays so slowly, you will probably fall asleep while playing it.
-James Bond
Really lame game with ancient graphics and ugly, repetitive levels. You don't even get to run around and act like Bond. It's just a vehicle-based game.
-Lode Runner
I love many Lode Runner ports, but not this one. This is one of the ugliest games on the system, if not the ugliest. It also lacks good level design.
-The "Clone" games
Pacar and Sinbad Mystery are Pac-Man clones. Congo Bongo is a Donkey Kong clone. All three of these games blow big time, although Congo Bongo for the SG-1000 looks a lot better than the Atari 5200 version.
There are a lot of other games that I think fall somewhere in the middle of good and bad, like the several racing games for the system.
I'm not passing judgment on the couple of games where I couldn't figure out what the Hell to do, like Pop Flamer, and the archaic RPG called "Black Onyx."
I'll also briefly comment on Dragon Wang, since if you're like me, the title alone makes people curious. It's a very cool concept. It's like Kung-Fu, with platforming, and more variety of enemies. It had the potential to be the very best game on the system. But it is insanely hard (Hehe, hard wang). The few times I've played it, I go for about two minutes, then I make it to the first mini-boss guy, and he slaughters me. If anyone can master this game, you'll have a blast, I'm sure. I can't play it though.
Hopefully, one or two people find this information helpful. Even if you don't find it helpful, perhaps you'll reply anyway just to make me feel good.