If you have played MechWarrior4: Vengeance then this is more of the same, but better! The missions are more varied, some are quite short while others are more epic, and the campaign itself tries to be semi-nonlinear ( you can choose the order of the missions to some extent, and skip out a few ). Of course since the engine is essentially the same the graphics, while clean and clear, are definitely a few years old – no directx9 effects here.
Replayability is offered through a split in campaign course but it only differs in about the last three or four missions. There is also the option to select different Mercenary bands as “sponsors” which affects what equipment and mechs you can buy early on but it doesn't make a huge difference unless you actually want to use the some of the more wierd weapons available.
Special mention goes to the Solaris part of the game which involves gladiatorial style fights and it probably the simplest part of the game for just blasting around.
Multiplayer isn't bad with a number of custom maps out there at the moment and even a few custom mechpaks (free ones, not the purchasable mini-expansion packs), however good luck finding a server in NZ (theres probably one or two in australia tho).
Overall worth the purchase if you like Mechs at all, if you are more of a FPS fan or really want a more console-type get-in-there-and-blast-it game then maybe this isn't for you (much of the fun is in mech design and moving your fellow mercenaries around).