I bought a 21st Century M5 over eBay last winter (used) and had to replace the battery right off. (Not the fault of the tank itself).

The 21st Century product is Chinese, and like Heng Long, they're mostly plastic and are vulnerable to any kind of abuse (my definition of abuse is anything off-road). The advantage, or trade off, is the price, of course.

I found that it can't take much rough terrain -- grass is fine. Snow was a big problem because it got caught up in the idlers (frozen, icy) and caused the track to come off. Also, it made the plastic brittle and the tensioner spring housing broke as a result. It also picked up small stones and would cause binding at the idler and track return rollers.

Another weak point is the (front) drive sprocket -- the plastic mount holding the retainer screw broke on one side but I was able to repair using platic cement.

Once I knew its limitations I stuck to concrete, grass, and generally easy terrain. I also found that they're easy to modify using a dremel cutting tool (removed the armored skirts).

The only high quality M5 I've seen is from Dave Spring. See http://6tharmor.netfirms.com/tank.html to view his all-metal M5. Very expensive but the only all-metal, high quality M5 that I know of.

Cheers....