It depends on what you want to make parts for . . . I've seen RipRap parts and the accuracy is horrible. If you want to do it for fun or as a learning experience, go ahead - it's a huge project on it's own. But don't do it with the expectation of good functional parts for robots.
The MakerBot is much better. You can't do high precision stuff, and materials are limited, but for making neat little thingamabobs it can do the job.
That said, keep in mind you can also pay to get 3D parts from *quality* 3D printers, too. An interesting part will cost you about $50 including shipping, with big discounts if you order many at once. If you want functional parts for robots, I recommend this route.