Tor multiply encrypts all traffic passing inside of it (see "onion routing"). The only Tor router that sees your data without encryption is the last one you bounce it through, and an evil router only has a 1/400 chance of being this router.

Anyway, this probably isn't any worse than regular Internet use, where every router from you to the recipient can read the content of the traffic. It might be worse if you expect the Tor routers to be less trustworthy than the random Internet routers your traffic gets routed thrtough. I am of the opinion that it's probably better, since with Tor at least they don't know who the sender and recipient are. This may be more damaging information for some applications (indulging your tentacle porn fetish) than for others (webmail login).