Speedbird,
Don't listen to the miserable, bitter, you've-got-to-be-committed, bunch. I did exactly what you're thinking of doing. Joined the RAF on a Short Service Commission, got an ATPL, was going to leave and join an airline - but then realised I was enjoying it so stayed in (joined in 85). Still got the ATPL, still use it on occasions.
If you're committed to flying you will succeed whoever you work for.
My advice: Get in, get an ATPL, if you like it stay in, if not - you've got an ATPL.