My first machine was quite similar in looks to this gantry.
I used steel tube though. I did use aluminum on some parts too, so it's a combination machine.
It is a small router but is still going great. The basic design works very well really. You don't have much concern with the Z tilting front to back under cutting force like some lighter over under rail gantries.
The trade off is accessibility to the router.
I used two cam clamps and pop my router out for bit changes.
Works well.
Here is a link to that build that may give you some ideas on what to and not to do.
http://www.cnczone.com/forums/showth...2&page=1&pp=40
Below my Z carriage, I used one double roller bearing and three delrin pads with eccentic adjustment. That too has worked well. V rollers with cam adjust would be a good choice too I think. Maybe a bit more precise.