How much am I going to have to budget for a decent mountain bike to hit the trails. New or used. What are the good brands? What are the bad ones?

kind of a open question...
What's your budget?
Where you planning to ride?
There are lots of good bike companies, and good companies who make some terrible models.
Avoid bikes in Walmart and Target.

I'm back to mountain biking after a 25 year absence. A lot has changed... Agree with above, check out bikes in a bike shop if you can. My GF and I had to wait for 2 hours to get into ours on Saturday, but she came out with a new bike so it's all good!

I'm shopping for a new one myself, as all the bikes I'm used to are now labeled "vintage," which makes me a little sad...haha.

Guessing you'll want something for medium to light trail riding? So maybe you won't require rear suspension?

Head to your local bike shop! As stated above - we need some more details.

Bike shop good.
Walmart bad.

Asking budget is a loaded question. My friends who don't bike think its crazy for spending the money that I do. But with that said, its my sport. I also understand buying cheap parts means more repairs and less performance.

Also, full suspension or hardtail?

Bicycling magazine recently reviewed hard tails from Trek and Giant under $800 which were really capable, nice bikes, just as an example. I think many brands probably have competitive offerings in this range. My LBS is a Kona/Rocky Mountain dealer, both have really sweet rides at the bottom range of their lineups. I ride a fully rigid Jones Plus, you can get a complete version for $1799 if you want a terrific do-it-all bike but they're not for everyone.

+1 on the LBS - the brand doesn't matter so much as how it fits and what components it comes with.

If you're looking for ways to save money don't worry about full-suspension, you don't need it and it adds cost and complexity. Low cost components are fine because they're often just as good as their more expensive big brothers, just heavier and you can always upgrade down the road as they wear our or break.

That's my $0.02

I prefer to ride a bike with full suspension. However, most of my bikes only have front suspension, so I can put a rack on the back and carry things. Even just going for long rides, I like to carry a large amount of drink. Having a rack enables you to carry that easily.

Obviously, different people have different preferences, and can decide for themselves what they like.

When buying a bike, I sometimes buy the bike that I get a good deal on.