Congrats! Harnesses are awesome! I'll never forget buying and using my first harness.
So, the pros (I can't think of any cons):
Using a strap-on to anchor the toy to your hips (or other parts of your body, since they make them for thighs, foreheads, chins, etc) is a very different experience than using it with your hands. Don't get me wrong, I love using my hands. But the strength and stamina of your arms will never compare to what you have to offer from the hip. We're talking about using the largest, most powerful muscles your body has to offer.
Plus, when you're penetrating with your hands or a hand-held toy, you are generally limited to keeping your upper body between your partner's legs. With a strap-on, you're connecting the center of your body to the center of theirs- you can be face to face if you so desire, with your hands and lips free to do wonderful things.
Annnnd you get to hump/be humped! Humping is just plain fun and feels awesome. The bottom obviously gets the bulk of the stimulation, but the friction can feel very very nice for the top as well.
I could go on and on...
A few things you'll want to consider when choosing a harness:
One strap or two? For the wearer, it's basically the difference between a jockstrap and a thong. Because the two strap is anchored to your thighs, it provides great stability and control, plus it leaves your own bits more accessible. I've never used a one strap, so I can't really comment on them. If you're used to wearing thongs, I imagine it would be just fine.
Materials? Lots of options- leather (sexy, requires special cleaning/conditioning) , rubber (shower/pool friendly), nylon (usually machine washable). Regardless, I'd look for something made with quality material, as a good harness is an investment that will last a long time with proper care. Watch out with rubber especially, as it should be thick and sturdy, not thin or plastic-like.
You'll want something with adjustable straps for a good fit (D-rings or buckles, D-rings are the most versatile) with a removable O-ring (ring that holds the cock, you may want to play around with different cocks so you want to be able to switch O-rings).
A note on cocks- cocks that stand straight up or at a slight curve are great when you're holding them in your hand, but when they're strapped to your hips it's a different story. Something that curves way waaay upward, like a flesh erection, is far preferable for positions where the top is horizontal. If the top is standing or kneeling behind, cocks that stick straight out are fine.
So yeah, hope that wasn't too overwhelming. We've barely scratched the surface, so feel free to ask more questions