I do my own 300 sq.ft sections, if not in a prime location where it needs to match the surrounding area, like a front entrance leading up a vaulted stairway...
Bedrooms, garage, kitchen ceiling (with flat paint and track lighting to obscure the "errors" after dropped ceiling light removed).. easy.
300 sq.ft is about my limit, though. We definitely hired out for larger jobs, and it is so worth it. The professional crews are very fast, excellent quality, and compared to other handywork, cheaper by the hour. Cheaper here than painters, even.
If you have the extra time, just adding more and more mud and sanding it all down to nothing with huge piles of dust, will make the work turn out well for the beginner. No glossy paints though!