It’s been my main camera for quite a long time, until a Full Frame D700 came along. Later when I sold the D700 in need of cash, I felt like trying something new and went mirrorless with the Panasonic GX-1, then numerous Sony NEX cameras. I could never quite go back to a DSLR once I got hooked to the small hybrids, so the D90 has basically been sitting on a shelf ever since. There is, however, a situation for which mirrorless cameras don’t cut it yet : action shots. If you’re trying to nail birds in flight or racing hounds or jumping horses (or a million other fast moving targets, young kids being another fine example), the contrast based autofocus system or mirrorless cameras has trouble keeping up. m4/3 cameras are much better in this regad that NEX, NX or FUJI X, but still not on par with real DLSRs. So whenever I head to the stables with the plan of shooting jumping, the D90 comes along.
I’m not saying NEX canNOT do it. They just can’t do it as well and naturally and painlessly as DSLRs do.
The main difference a 5 yr old sensor has with current ones is lower DR and lack of live exposure preview. Both things make me opt for RAW + jpeg on my Nikon. Whatever exposure niggles I can’t anticipate due to the lack of an EVF are easily dealt with in post processing if needed.