Giant use good stuff on their bikes, and the Roam 3 is a very solid value. The drivetrain is on the lower side of middle class, but it works fine. The front derailleur is a Shimano M191, which is from the Tourney group. The rear derailleur is listed as "Acera", and is probably an M360. It uses an 8-speed Shimano Hyperglide HG31 cassette, and a Suntour triple crankset. My Trek uses an identical drivetrain, and it's smooth and very functional. Not top-of-the-mountain, but fully serviceable for this type of bike. The Roam 3 has a basic shifter/brake lever combo set on the handlebar that will work well. The disc brakes should also be fine; they're basic mechanical units.

The only thing I wish this bike had that it doesn't is an adjustable quill stem vs. the threadless headset and stem that it does have. If the handlebar height is not to your liking, you'll need to buy either an adjustable stem or at least a stem of a different angle/length to suit your geometry. Adjustable quill stems afford the user some adjustability without having to buy additional parts. Given the price point, though, you'll be able to afford to replace the stem if you need to.

If you're not comfortable working on bikes, yet, then a used bike is probably not the best option. Or -- at least not the best option for your primary bike. You could always buy something like this Giant and start riding, then pickup a low-cost used bike off Craigslist to just start wrenching on.