Hi Darine!
I used to teach two LesMills programs at a local gym, Body Attack and Body Step. While the classes are fun and got me really fit for a while, they get played out over time (this is just my opinion). Let me explain...
The programs are all pre-choreographed and set to specific music. I would receive my instructor DVD which taught me the moves and it also came with a CD that I would play in class while teaching the moves I learned. Body Attack always has 11 "tracks" with a specific purpose (ex- track 1 is the warm up, track 2 is mixed impact, track 3 is aerobic track, etc.), and Body Step along with their other programs are the same way. The music and choreography changes every 3 months so after a while it gets boring as their is no variety. Even when the music changes, the choreography only changes a little. They are good classes to push your stamina, don't get me wrong. But they don't offer any variety like Cathe does. You will never do "steady state", "interval", "circuit", like Cathe offers, but rather the classes will always follow the same format, therefore your body will adjust after a while and you may not continue to receive the same fitness benefits.
As for the company, from my experience they are way too over-the-top fitness fanatics for me. (No offense to anyone who considers themselves a "fanatic", if you were on the inside of this organization you would know what I mean!) Every three months I would have to attend something called a "Quarterly" where we would get a preview of the new music/choreography set to be released. We would be taught by Les Mills "Master Trainers" (people who travel around the country to train instructors on how to teach the classes), and these people performed like rock stars and were worshiped like gods.

Picture a rock concert with booming music, fog machine, costumes, screaming "fans", etc - but for aerobics!! The whole thing was very scary to me. My group fitness manager expected it to be my life and I just couldn't deal with it anymore. To me, Les Mills is very "cult-ish". Just my opinion! Other LM instructors may have a different opinion, it just wasn't for me!
Here is a link to their website so you can learn more about the company and the classes:
http://www.lesmills.com/
Overall since you are new to LesMills, I think you will enjoy the experience of taking their classes for a while! It's fine if you're just a participant, take the class and then go home.
Also, I know they sell the instructor DVD's on EBAY, but they go for a lot of money.
Good luck and have fun!