I've filed once.
I was flying a microlight off a beach - non-radio, enjoying the view. A PA28 came along, flew alongside me, then turned directly towards me forcing me into a steep descending turn - fortunately with no nasty outcome. I was easily able to read his registration, and filed an airprox.
The pilot, who I imagine had a lengthy chat with his club CFI later without tea and biscuits, reported that he wasn't looking out as he was receiving "flight information" from an airport 30 miles away, and they had reported no traffic.
A moment of humour was that I reported my altitude on the QNH. The airprox board chappie phoned me up and asked where I'd got my QNH value from as I'd not flown from an airport, was non-radio, and there was no record of me contacting any airport.
"Well, I took off from the beach at low tide, and it read zero halfway between the high tide and low tide marks".
He went silent and thoughtful for a minute, then commented that there *might* be something wrong with that, and he'd get back to me. He never did.
In recent years - I write having sat in on one Airprox Board meeting as an observer, and read a reaosnable proportion of the reports, it seems to me that the main deficiency with the system as it presently is, is that whilst they try fairly hard to analyse the causes of problems, there's a massive reluctance to issue any kind of recommendations that might reduce them. Taking that leaf from AAIB's book would seem to me to be an improvement.
G