I tried Retro/Grade, and it makes sense, but there isn't enough variety in it to make it visually interesting, and the music isn't addicting at all.
Space Giraffe makes a LOT of sense when it clicks, if you don't understand the game, you might hate it. Essentially you're trying to keep yourself alive until you can "bull" off the enemies for a score multiplier. You can shoot the enemies, but that's less of a score, but required to stay alive against bullets and flowers and possibly other enemies. The key is avoiding bullets and flowers(shooting them beyond a certain point), then bulling the normal enemies. There's a demo on the official site, give it a try. For $18? It's either $5 or $10 on the XBLA, so I'm not sure. Not that it's horrible, but the price difference hurt Llamasoft in this case. Gonna give a wait, but might give in before the sale is over.