Over spring break I spent a good amount of time playing 2v2 with my brother who had recently bought SC2 and was playing it for the first time since beta.
2v2 is of course quite a different creature than 1v1.
As a double Terran team we eventually fell into a strategy where he would tech to mass banshee as quick as possible and I would mass marines with every upgrade possible. It usually plays out that I hold off early attacks with ungodly amounts of marines while my brother goes and shuts down the economy and tech of one of our opponents. It's fairly effective since the enemy is almost always out of position and can't return from their attack quickly enough to defend.
This is similar to our 2v2 beta strategy which usually involved me going M&M&M while he massed vikings and dropped into the enemy mineral line.
Fast-forward to now and I find that I've adopted the strategy of mass marine in my 1v1 matches. This invariably leads to hilarious meltdowns on the part of my opponents who curse me out for using what they deem an unskillful army composition.
For those of you that have read Playing to Win, you'll likely agree with me that the rules of a game are impossible to break, and so any strategy possible is valid (save for if it literally makes the game unwinnable, as in it's frozen).
Even so, mass marine is far and away from being an abusive or imbalanced composition. It suffers many of the vulnerabilities shared with any bio-based composition. Tanks, Psionic Storm, and especially the new incarnation of Fungal Growth can absolutely shut down a crowd of marines with even the most minimal of skill.
The composition's strength lies in exceptional mobility (especially when paired with Medivacs and decent multitasking to achieve multiple simultaneous attacks on different locations), potential for strong early aggression, and cheap production cost. With upgrades and good micro, marines can really clean house.
Despite performances the likes of MarineKingPrime, I don't think mass marine would hold up in higher level games. I'm going to keep playing it until I have a very solid build and then search for alternatives. It will serve as an excellent fallback in matches that warrant its use, and there's little more amusing than seeing 125+ marines (save for maybe 150 banelings).