I drive four shetlands in single, tandem, pair, unicorn (three ponies in harness, two behind and one in front - not the mythical beast!) and will do four-in-hand as well soon.
I drive - a pair of cobs, a pair of black ponies (one oversized shetland and one dartmoor) and they all drive single too. The cobs have also been team leaders and one drove unicorn in his last home.
I've just had a wee go at long lining (not driving I realise) and found it quite fun and interesting for my horse. He constantly surprises me, I thought he might find the lines a bit scary or get upset at them around his bum or my bumbling attempts at turns - but no, he took to it like a pro with barely a blink. He was interested in the concept of going for a walk with me behind him and generally had a fun time. I think it'll be great for his confidence (and mine! not having to deal with his shying onboard)
at my riding school most of the horses are driven so there is an oppurtunity to try but im not really interested. I think it looks cool but i dont think i'd be any good at it