They're tough, that's for sure. Yes, I can do them, but I doubt I could do more than 20 interspersed throughout a class. Ultimate Yogi has a lot of them; I started out just doing 2-3 throughout a practice, then worked up. The caveat is, I developed an awful case of elbow tendonitis that made me put a stop to all yogi pushups for about three months. So, don't build up too quickly, there's really no great rush to build strength in a yoga practice. Trying to build strength too quickly, just leads to injury in my experience.
Also shoulder alignment is very, very important in chaturanga. A good instructor will show you what to do, but no one is there to correct your form with dvds. So, we've got to be our own instructors: Here's a great article; I've read it about ten times and still correct my form; hopefully one day it will become second nature:
http://www.yogajournal.com/health/2528