I looked at those at one point and my concerns were the risk of side-to-side movement as they are only clamped to the seat post. Also size, weight, and fugly.
If you have the standard rack mounting points, I see no reason to look at anything like that.
Anyone any experience of the Topeak MTX Beamrack setup?
A mate has offered me his - looks like the capacity is ok but it's a fair lump, any better than a lightweight rack + pannier?