I'm putting it off cause I've already got a really good channel strip in the Sebatron Thorax and the pres in my RME are really nice and clean, so I've got five channels of really good pres already. I do 99% of my EQ and compression ITB with the McDSP plugins so I don't really need that outboard.
Most of the work I do is either recording bands for someone else to mix or mixing stuff other people have recorded. For the former I just try to get it sounding good with mic positioning and then give them something neutral to work with, for the latter I would have to run stuff out and through another DA/AD stage to do it outboard and I'm not convinced the benefits would be worth it.
When you say a decent investment what do you mean? Like something you can sell later on and make money? Don't think so. It would be a good investment if you could get more work just by having it than you would if you didn't, or it made enough of an improvement to your tracks that you increase the money you're making by more than the amount that it cost.
I reckon that 90% of 500 series stuff is sold to people who just want really nice stuff. Nothing wrong with that, that's why I will want it. But as it's a luxury there are a bunch of other things I'd be better off getting before hand, like mics, this digital desk I've got my eye on, some software upgrades, a small PA system, a new car, getting merch made, holiday to europe in June etc.
When I do take the plunge I'm looking at a bunch of http://classicapi.com
stuff, the plan is to basically make myself a little 8 channel modular mixer. To use as a summing mixer with EQ but mostly just as a little project to keep me happy. I don't fool myself into thinking that if I have that all of a sudden everyone will start buying heaps of my music of people will be throwing money at me to mix their records, but it will still be nice.