A couple days ago, there was a poster trying to decide between two budget 3D monitors. One was a B-grade (I don't know what B-grade actually means) active shutter system, and a passive system. He ended up deciding to get both and send back the one he didn't like as much.
He updated us with his first impressions of the passive system, and he comments where, "The text is so awful".
I've seen people who like passive systems, as they have no ghosting, and are brighter, but text is bad with a passive system, as not only is the resolution halved, it loses every other horizontal line, which makes is worse than 720p compared to 1080p.
The brightness issue on active shutter systems are being addressed with new standards like 3D Vision 2. Even before that, monitors like mine increased brightness levels to the extreme to help counteract it the darkness issue (I have my monitor set to 30% brightness when not playing 3D Vision games, it's at 100% in 3D Vision, and 30% is still quite bright in 2D).
HD3D is ok, and the software support is just about as good as 3D Vision, but HD3D is a little slower to support games and have been darker than the newer 3D Vision monitors, and their software is not as easy to use. I'd recommend getting 3D Vision 2.