In reply to The Lemming:
4/3rds pros and cons:-
pros:-
Size - everything is smaller and lighter 'like for like', especially lenses.
Increased depth of field - if your style needs more stuff in focus then this is good.
Increased 'reach' - a 150mm lens on 4/3rds is equivalent to a 300 on FF in terms of angle of view.
IBIS - olympus stabilisation is the best there is and compensates for the noise disadvantage below (for static subjects).
lens adaptability - you can put pretty much any lens on them with an adaptor due to the short flange distance.
cons:-
not so good in low light - the smaller pixels suffer noise more than bigger ones in FF sensors. If you're shooting in low light often e.g. astro then results aren't as clean as those from a bigger sensor.
Dpeth of field - if shallow DoF is your thing then it's cheaper and easier to achieve with FF.
EVF - no m43 cameras have optical viewfinders.
Lens range - you can't get native tilt/shift lenses.
continuous/tracking autofocus isn't as good as that on a half-decent DSLR.
battery life not as good as a DSLR.
So, analyse what you shoot and work out what you need to shoot that sort of stuff best, then try some cameras and make a decision. Unless you're printing large images the image quality from M43 is good enough.
Video - I believe serious video shooters use Canon 5Ds or Panasonic GH4s these days unless they have a dedicated video cam. Olympus IBIS is great if you're shooting handheld all the time but their video options aren't great unless you've got their latest models.
Personally I use an Olympus E-M5, it's the first 'proper' camera I've owned so I can't really compare it to other things.