It's an entirely subjective question, depending on what you want to do in life.
If you want to work in a bank, or go on in academia, then you'll need it. Same goes for anything remotely medical and/or research based. There's no other choice.
For me, it's most definitely an investment. I did my undergrad (thankfully at the old fee caps), then a self funded masters. If I didn't have a student loan, there would be no way on earth I'd be able to afford to do that. I'm now doing a PhD (luckily, research council funded) - which wouldn't have been possible without my prior degrees (and therefore, without the student loan). I don't really care that I'll have a, quite frankly, tiny amount deducted from my salary for the foreseeable future. The knowledge, experience and colleagues I've gained over the past few years are priceless in comparison.
I do agree that far too many people get pointless degrees these days, though. One of my first lecturers said to me that "you're better off dropping out of a good degree from a good institution, than getting a first in a crappy degree from a poor institution, simply because it shows that you had the ability to get on the course in the first place."