Pantheon is pretty good, especially since a bunch of people can't see his ultimate ring due to a still un-fixed bug (FIX IT RIOT GAAAAAAAAAH)
Anyways, he doesn't play like most AD carries. He hardly ever autoattacks, and is more of an AD caster, where he stacks AD but does most of his damage through his physical damage spells. He has a strong early game, and if you can deny creep, harass, and zone whoever you're laning against, Panth would be a good asset for a team to have. His ult gives him great map control and ganking potential, and while he's somewhat squishy for a melee champ, he can do a ton of damage very fast.
Build a Doran's blade first, and feel free to stack doran's blades if you think you can gain a decent advantage by it. I usually don't stack more than 2 or 3, but I've seen high elo players go up to 4 before they start building other items. It's fair to say that if Panth can't control the early game at least a little bit, it would be better to pick another champion, so really try to exploit his great early game strength with doran's blades as much as possible. You should get boots 1 relatively quickly (some panths get them first item), and get Last Whisper and Bloodthirsters as your core for late game.
Put enough levels in Spear Shot to harass, but you should be focusing on your heartseeker strike. Imo it's still not necessary to put too many levels in Aegis, but as the stun duration does increase with levels, put as many in it as you feel would benefit you.
Panth is also one of the few champions where I don't level his ultimate whenever available. Most times I just get 1 lvl in it at 6, and max other abilities first, since leveling it will only increase the damage, and usually people can move out of the center quickly enough that its damage won't mean as much as an extra lvl in Aegis or Heartseeker. With the circle bug, that might be different, but it's something to consider.