giant robots

If you don't have MW4 this game is useless. The real review to read is there. If you liked it, THEN read this. The mech pack contains 4 new mechs all of the Inner Sphere (duh).
Hunchback: .... Too little tonnage for its load out
Dragon: Performs like a medium mech on weapon steroids
Zeus: A very strong and well-rounded assault mech
Highlander: Great load out with jump jets!

It includes the coliseum, which is a very fast paced and good map. It has lots of interesting obstacles like arches and a secret cavern.

The Heavy Gauss rifle is very strong, and can fire 600 meters. But like the name says it's 18 tons!

And finally the outrage of the expansion. The IFF Jammer makes your mech appear neutral to the enemy when further than 400 meters. So what? You shoot a neutral mech.

Over all it's a ... package that brings a little fresh blood in.

*Addition*
This has been confirmed as compatable with MW4:mecs. Ten new mechs is a lot, but if you play the campain and not multiplayer (I play multi more, but the camp a lot) then these meckpaks will bring in 4 new mechs each and 1 new weapon and 1 new electronics. There's always the 1 map that wasn't advertised(Gagetown for IS, New gothem for clan). But both advertised maps will be included in base Mercs. With both paks 18 new Campain Compatable Mechs. That's a lot. The base game of MW4 had 21. It's worth the extra [$$$] for both.

The Inner Sphere pak is great!

It's what a lot of MW4 players have been waiting for, and it doesn't dissapoint. The IS pak comes with 4 new mechs (Zeus, Dragon, Highlander, and Hunchback), 1 new weapon (Heavy Gauss), 1 new component (IFF Jammer), and 2 new maps (Coliseum, and GageTown).

The IS pak also comes with the 3.0 patch and it fixes A LOT of bugs in the game that cause certain mechs not to take proper or any damage.

You NEED Mechwarrior 4: Vengeance to be able to install the mech pak. You can use the mechs in instant action and in multiplayer only. You can also play others who DON'T have the IS Mech Pak yet (they have to have the 3.0 patch too), but they won't be able to choose any of the extra stuff.

CONS:
The only thing i didn't like about this expansion was that it didn't have any new missions like MW4: Black Knight did.

Some people have said that Instant Action doesn't work too well with this new pak, but i haven't had any problems. I think they fixed the problem with your team not respawning in Instant Action when you have more than 1 life, but the catch is they respawn after you get destroyed. For each life you loose, you get your partners back.

Once you install MW4:IS you won't need the CD for a while, because you can't run the game from the disc. You can only run it from the Mech Warrior4: Vengeance CD (either cd1 or cd2 will work). That makes it seem like a waste of money though.

Don't expect to see a highlander running around in the MW4 Missions.

I have not had time to enjoy MW4 online with the pack, but I like what I have seen. I hope that MW5 debuts soon.