Alphonsus

What are your opinions?

Speedway2

Op.....in your price range I would consider a "lightly" used one vs buying new.
When I was looking around for a MTB I had to consider bikes costing ~$1000CDN to get something half decent new.
In the end my needs and actual use led me to a rigid "Hybrid". Great option for mostly paved roads and some light paths and well under your budget.

Good luck with your search......

Dirt Farmer

Yes, buy used on Craigslist. Plan on spending even more to service any suspension (if you buy a suspension bike).

TiHabanero

500-600 bucks will buy a mountain bike looking machine good for light trail use. If buying used, do not go full sus unless you are willing to service pivots. Suspension forks and rear shocks are another issue to contend with.

Unca_Sam

If you haven't figured it out, your intended use will drive the price. A full sus BSO is $130 on craigslist here, yet I realized that my preference is going to be XC or singletrack, not all mountain, gravity, etc. I picked up an early 90's rigid frame, because I figured it'd be lighter than a similar level hardtail. I'll have to refurbish it, but I'm OK with that. Fully functional specialized and trek frames here are around $200, with and without a squishy front end. Full sus alloy DH machine? lightly used it's still over $600.

Brocephus

"Good","Better", "Best", doesn't usually become real apparent until somewhat higher price ranges. At the $500 dollar range, most bikes from a given name-brand manufacturer are usually going to be pretty comparable (and just barely above what might be considered outright junk).
As mentioned already, a clean, low-miles bike on Craigslist is probably your best bet. Also, as already mentioned, suspension gets you into another arena entirely. If you expect to do any heavy riding that will really need suspension, I would advise you save more money, and up your budget for a better bike.
If you plan on any serious use, you want a front fork that costs at least as much as you're wanting to spend on a whole bike !! Cheap forks are junk, and don't hold up, except for road and light trail riding, in which case, they're basically just a plain front fork, but with extra weight, and still having potential maintenance headaches.
I've been digging the rigid fork/hybrid concept lately, in fact I just sold my trusty old trek 26" mtn bike, and a couple months ago, built a rigid-fork, hybrid/dual-sport type bike, with 29" wheels, single speed crank, and 9-spd gearing. That bike will be fine on trails and dirt roads, but is also pretty nimble on the road, as a commuter/exercise bike.
I'm older now, and have no interest in thrashing the hell out myself and my bike, off-road, so I don't need the extra weight, cost, and maintenance headaches,of a suspension bike. Makes things easy, and i have a better bike without suspension, than I'd have if I spent the same money, on one with suspension . This may or may not apply to you, but you get the idea.
So, what type of riding to you actually plan to do?

Darth Lefty 

For that budget I’d buy a rigid fork hybrid, not a MTB.

If I wanted a MTB I’d budget north of $1000 for a bike with a hydraulic damping fork and a dropper post. Probably the bottom build on a model that goes up near $2000.

I haven’t yet run out of nice things to say about my Timberjack.