
Here is a quick DOF comparison. I shot my 45mm Olympus f/1.8 lens at f/1.8, f/2.2 and f/3.5 to show the visual difference between what people say the MFT equivalents are. Frankly there is little between APS-C and MFT.
I would argue the equivalent isn't quite right either, as the format size difference doesn't take into account aspect ratio at all. We all know that a full frame camera can achieve thinner depth of field, but when I
keep reading in message boards that the 75mm f/1.8 is 'only' f/3.5 in full frame DOF terms they make it sound like it has DOF to infinity. So here is a visual example. You sort of need to think of it as MFT on the left, APS-C in the middle, and Full Frame on the right.
Much more important to me is the lens you have with you. When I shot APS-C I'd bring my f/4 24-105mm zoom with me, since I could carry that on a strap while the larger primes I had didn't slip into pockets (more weight and volume). It isn't that I couldn't carry them in a pocket - the point is that I DIDN'T. With MFT I'm much more likely to actually have a fast prime with me, so in many ways MFT gives me a lot MORE DOF control that APS-C. Full frame is another story, but I've never considered getting into $3,000 bodies and L series fast primes. What is possible, and what you can afford are two different things.
A big plus of MFT is that you can get a very compact body to add to the system for less than $400 new, or in the $150 range used. No such choice in Canon/Nikon land.
Don't even get me started on equivalence - of course Full frame gathers more light and has less noise, but does it make a difference to your photos? If yes, then consider full frame, if not then be happy :) I see so many people arguing how important these differences are that can't even get a sharp photo, let alone something with pleasing composition and an interesting subject. Whether I do is kind of besides the point - I'm arguing that we can be happy with cheaper, smaller cameras not that everyone needs Full Frame (or bigger).
It isn't necessarily a good thing, in and of itself, that MFT offers less ability to blur backgrounds, but the reality is that we work with the tools at hand. There is no question that one can get creative photos with MFT cameras.
Is the Olympus 75mm f/1.8 too expensive for an 'f/3.5 full frame equivalent'? Who knows? We each have to look at the benefits and the price and weigh them over. If it isn't of value to you, then don't buy it!