I shot with a Canon 1DMarkII and 5D with a bunch of primes and L zooms.
I was an early adopter of the micro 4/3rds system first with the Panasonic G1 then with the Olympus E-PL1. I never intended nor expected micro 4/3rds as a replacement for my DSLR system but rather an alternative. On one hand the cameras are much smaller and so are the optics. My entire system is about 25% of the size and weight of my Canon DSLR system. My ONA Bowery bag (very small) that can fit my OMD E-M5 + 12-35mm f/2.8 + 35-100mm f/2.8. A bag that could not hold a 5D with a 70-200mm f/2.8 alone. No 600mm DSLR zoom exists that is comparable to the size of my 100-300mm. It is a crop sensor camera with PDAF implemented. With that comes some other compromises, such as high ISO performance, and AF tracking. Several members here and other forums have asked me similar questions and I always explain both sides of the coin with the final message that, one, its not for everyone, and two, many will not see this as a replacement for their DSLR. On one such discussion, I described it as the system the working professional uses on their break or personal time.
It-All-Depends-On-Your-Preferences-And-Expectations.
If size is a big deal... the micro 4/3rds is the sweet spot in my opinion. An ideal blend of performance and packaging (size/weight). No other system has the wide selection of camera bodies all with different designs/interfaces to select from. You can go as small as an E-PM1 or as large as the newer OMD E-M1 or G6. For video, you have the new Black magic camera as well as the GH series bodies.
If high ISO performance is a big deal... then simply stick to the FF DSLR systems and accept the sacrifice of a larger, heavier system. Same deal with DOF... heck consider MF.
As I said, I never intended and still can see the benefit of owning both my Canon system together with my current micro 4/3rds system. Except that I no longer shoot or have any intention of shooting professionally. My focus is my life and growing family and the micro 4/3rd system allows photography to blend into it rather than attempt to compete with it. As such, I shoot more... very important. I ended up selling my Canon because it was gather dust. My other system which is FF still remains though.
I still have and shoot with the E-PL1 but it is now joined with the OMD E-M5. The EM5 improves on sensor performance and AF. Shooting sports is still not as effective as my 1DmarkII but I've learned to work it with specific settings AND taking advantage of the EM5's very fast single AF... pump focus frequently. The newer E-M1 will be interesting as it hopefully improves on these issues further.
Of course there are other mirrorless cameras, in particular the Fuji cameras, that are interesting and equpped with APS sensors... but in general that means larger optics as well.
As for lens recommendations.... there are a lot to choose from although fast telephoto primes are still yet-to-come.... I recall two already announced.
I like and shoot with the following:
Panasonic 12-35mm f/2.8
Panasonic 35-100mm f/2.8
Olympus 9-18mm
Panasonic 100-300mm
Olympus 12mm f/2
Panasonic 20mm f/1.7
Olympus 45mm f/1.8
Other highly recommended that have peaked my attention.
Olympus 75mm f/1.8 (portaits...)
Olympus 60mm macro
Panasonic Leica 25mm