As an engineer, I've not yet had to deal with a commercial aircraft that has rejected take-off. I guess that commercial aircraft are maintained to fairly stringent standards in the UK, hence it is rare that anything goes wrong that is serious enough to warrent rejecting take-off.
I've seen three in light aircraft since I started flying a year ago:-
The first one was in a PA-28 taking off behind me. The pilot (from what I heard on the radio) forgot to secure the top latch on the door before take-off and the door popped open as the aircraft accelerated. Unfortunatly the PA-28 door isn't exactly Murphy proof (we all know Murphy's Law don't we?)
Number two, Cessna 172, the pilot (my friend) reported partial electrical failure. He lost his primary radio, ADF, a VOR and the GPS as he opened up the power, shortly followed by everything else avionic because he had to turn off the master due to a slight burning smell as he taxied in. The smell turned out to be the bonfire next door and he over-reacted!
The final one, another C172 we me at the left-hand seat, taking of from a remote private airfield in the middle of the countryside, a rambler with his dog thought the field was a public right of way and walked out into the middle of the runway.
To answer your question, I guess RTO's are fairly rare in commercial aircraft, but more common in GA/light aircraft.