It has nice stats, but like many other things, the Poison typing screws it up—it's weak to both Psychic and Earthquake, two very common moves in the OU metagame. Still, it can take special hits well and do some big damage with its moves, and it can be a pain with Wrap and paralysis support.
Tentacruel is one of those unfortunate Swords Dance users that can't OHKO Alakazam with Hyper Beam after one Swords Dance. If you can spread a lot of paralysis about, you should be able to do a lot of damage with Wrap. Unfortunately, Tentacruel's high Speed is still no match for that of the many faster standards, and it has no way of increasing its Speed or paralyzing anything, so it will need a lot of support.
Use Swords Dance against something that is asleep, or a Rhydon that's too cowardly to stay in, and if you are faster and can't OHKO, Wrap until you can. Hydro Pump 2HKOs Tauros, and also does a lot to Alakazam and anything else that doesn't resist it—except of course Chansey. Blizzard with max damage will 2HKO Exeggutor, but usually falls just short (Wrap can help with that, though).
Double-Edge is an every-turn attack that is boosted by Swords Dance, but Tentacruel doesn't like the recoil, so Hyper Beam is preferable. Supersonic and Screech can be used for support purposes. Mega Drain can work in UU to hit Omastar and Kabutops hard, or for just a straight out special attacker set.
Checks and Counters
Starmie, Alakazam, and Gengar are the biggest counters, since they outspeed Tentacruel and thus can't be Wrap-trapped. Tauros can switch in and counter it as well, and Persian can KO it more often than not with Slash + Hyper Beam. Pokemon like Snorlax and Exeggutor can beat Tentacruel one-on-one, but beware of Wrap, because unless it's paralyzed, you will either have to hope for a miss or watch your HP slowly whittle down if you don't switch out.
In the UU environment, Electabuzz is a good counter, as are most other UU Electric-types. Paralyzing Tentacruel is the key to counter it, because with his Speed and Wrap, it can be devastating.