kerala backwaters, and Varkala

Going to southern India for 2.5 weeks.
I was wondering how long people would suggest in the backwaters?
Books seem to say its a highlight of a trip to Kerala, but then I've also read you get bored there on a full day trip!
I was thinking of doing a three day trip, with one night on a boat, and one night at a "backwaters resort", with ATDC.
Any experiences, or comments on this? It is something we're willing to splurge on (within reason!), so if anybody has any suggestions of other tour companies, I'd appreciate it.
Also, I've read mixed reviews of Varkala - we were just going to spend a couple nights there, on the way to Trivandrum - any comments?
Thanks,
Deanne