Is the MEI a good investment? Well, I have one and other than the hours of multi I bought to get it, I have not used it. I haven't even been in a twin since my II/MEI combined checkride and there doesn't look to be any change soon.
Dashgirl actually made an interesting post in another thread about how FOs are stuck building a whole bunch of "worthless" SIC time and never getting that PIC time to finally go onto something better. Us CFIs are in a similar situation - building hours of single engine time and never getting that multi time needed to go onto something better.
I would say if there is the possibility of getting some multi students at your school, get your MEI - you can do it pretty quickly if you already have the hour requirements. The oral and flight are pretty straight forward. However, unless you know you are getting multi students, hold off and save your money. Most schools save their multi-engine students for the most senior people there, usually furloughed/retired airline people (there is another thread talking about if this is fair or not).
As TonyWilliams said, Pan Am was a great airline to work at, so was TWA and Eastern. Oops.
Word of caution: next to Union talk, 300 hour wonders, and scope, a very hotly debated topic here is the "Pilot Shortages." You can find several posts on front pages about one and there are always 2 arguments: It will never happen and never has happened when forecasted -OR- age 65 retirements and lack of students entering training will lead to a shortage. I don't think there will be a SHORTAGE, but I think there will be movement and modest hiring at acceptable minimums, ~1500total 100-200 multi with an ATP to be "competitive" at the regionals. But hey, what do I know? I'm just a CFI. Time to go spin up that Arrow.