Eh. I wouldn't buy one... MDF holding up a very expensive glass box full of water doesn't seem like a good idea to me. BUT. I've seen many, many MDF (or worse, particle board) stands, even ones that were wavy and have obviously been wet, with tanks STILL STANDING on them. Typically, they're 'good enough'. If they weren't, the aquarium makers wouldn't keep selling them.
My latest system, I had a small, local fab shop weld me up a stand, to my specs, out of 1.5" 11 gauge square steel tube. Took it over to the local powder coat company, had the mill scale sandblasted off of it, and coated in gloss black. Ended up with a STRONG stand that should last longer than I will. Has a much more 'open' design than you can accomplish with wood, and it still cost me less than the aquarium maker wanted for a stand.