I just got a few Romi kits and have had very good experiences with them so far.
The kit itself is very high quality, and goes together easily with few tools needed. It is surprisingly robust, and the physical tolerances are excellent.
The electronics seem a bit pricey at first blush, but when you realize that the Romi controller has the motor drivers and an IMU onboard, it is actually a really good deal. The motor encoders and the IMU are what makes the Romi a candidate for running serious WPILib code.
What really makes the Romi shine is the well-engineered Pi-to-Arduino I2C bridge. You could finagle such a setup yourself, but not in such a compact, electrically robust package.
So, to answer your original question, you could put together a system using a Raspberry Pi, an Arduino, some voltage level shifters, and a few sensors and drivers, and I expect that there will someday (soon?) be other WPILib-compatible platforms, but you would end up spending more money in the long run.
The Romi is a much better robot than you might expect from looking at pictures online. and having the ability to write real robot code on a tiny, inexpensive robot platform is truly a Game Changer.