As a kid I loved these books. Even then I realized the writing wasn't great but hell, they were going to Mars! Unfortunately I never actually got the last two books where they were actually on Mars.
Rereading them now I see that the writing is actually pretty bad. The kids' conflicts flare up out of nowhere with a few words and disappear just as quickly. There are lots of romance and other little subplots that really go nowhere. And the author had to keep it super clean so it's nothing like the way teenagers really talk.
Also there's no real wonder or excitement about being on Mars. They're just suddenly there working.