Ridiculous. RIDICULOUSLY HARD. But, awesome.
There are no short cuts in these classes (unless you have an injury). You can't do ANYTHING on your knees. I gotta say this kind of irritates me sometimes - isn't it better to do 5 proper push ups with perfect form than 20 shitty little quarter pushups?
Having said that, you're pushed to the max in these classes. Tony Austin is just as you picture him to be - he yells, he's tough, he makes you shout 'Hoo Rah' in the middle of burpees. He is perhaps a bit too rough at times...screaming in your face, (fairly lightly) slapping your arms. I could do without the slapping.
Classes are typically lots of old school moves (burpees, push ups, squat jumps), coupled with cool stuff I've never done before (crab walks, squat jumps while holding a 10lb bar above your head, etc.)
I agree with what Catherine L said - what's the point if you can barely do one rep? I think the instructors should offer some sort of modifications, but then - maybe I wouldn't work as hard?
Anyway - you'll get your ass kicked. I honestly wouldn't go if I was in bad shape - get yourself to a base line of fitness first.
Also - boxing classes require you to bring your own mitts.