I replaced my Kyosho ZX5 this year by a Tamiya TRF501X and haven't regretted it for a minute.
The ZX5 is a nice club racer, but in standard trim, it falls a bit short compared to full bread racers like the Yokomo, BJ4, XX4 or TRF501. We mostly race on high grip surfaces here (astroturf, grass) and the ZX drivetrain was a bit fragile. It easely stripped gears.
I can vouch for the TRF501X being a real TANK ! I've seen it survive crashes that would have broken any other 4wd. The suspension parts are so strong, that most BJ4 owners are replacing BJ4 A-arms by the TRF501 A-arms. The geometry is almost the same and it requires only a 3mm drill and a little bit of dremeling. The only real problem with the TRF501 is that in stock trim, the rear diff requires a lot of maintenance. This can be easely remedied by replacing the stock diff balls (ceramic aftermarked or plain old AE thungsten diff balls) and use some self-adhesive diff seals (I cut them myself from unused LRP-stickers