To quote the Wiki "Pinnacle Station is the eponymous station of the Mass Effect downloadable content pack Pinnacle Station."
It can be found in the Phoenix system of the Argos Rho cluster near the planet Vebinok and it offers you the chance to take Shepherd through some Holadeck style training missions.

There are four distinct mission types: Conquest, Hunt, Survival and Time Trial.
- Conquest requires you to capture control points as quickly as possible.
- Hunt requires you to kill as many enemies as possible in the time limit.
- Time Trial requires you to kill a set number of enemies within a set time
- Survival requires you to survive as long as possible against a never ending wave of enemies!

There are 12 missions to complete, three per gametype and if your character is of a fairly high level you'll need to turn the difficulty up to Hardcore or Insane to really give yourself a proper challenge.

When you are top of the leader board in all of the missions Ahern will reward you with a simulated version of a "real" mission he was on in the First Contact war. This mission is totally insane on the harder difficulties! You have to survive for 5 minutes while enemies continually spawn all around you.

I won't spoil it for those yet to finish it, but the final reward is really cool.

Overall I enjoyed Pinnacle Station despite the fact it does perhaps focus on a weaker element of Mass Effect (the combat). All the missions feel very samey despite the different gametypes and I think some more variation in enemies and challenge types would have been better. It's a great place to get any combat/bionic/tech achieves you are still to get though, since you can repeat the missions as many times as you like.

For Mass Effect fans it's a must buy at 400 points, but for those yet to try any Mass Effect DLC I'd probably point them towards the Bring down the Sky DLC pack first as this has a much stronger role playing element.