I have right now a 4 inch tall canopy and I don't like it. Too tall for me, and it covers all the tank., and although it can be opened, I hate having such a big thing over it. I am going back to my cheap DIY lighting enclosure ( http://<a href="http://www.plantedta...php?t=7019</a>
), it was more comfortable to work with that. I have also made a second one, and will place it on the front to have complete lighting. They are so light that I can remove them quite easily if I need even more space to work than they already allow. There wasn't any problem about height and light dispersion (even if so, most high-light plants are on the middle to the rear, and crypts use most of the front).
But that's just what I like.