After many years of having to use the coastal route to Caernarfon because of cloud on Snowdonia, a couple of summers ago I managed a Snowdon overfly in the Chippy. The scenery was quite stunning, and I was pleasantly surprised at the lack of bad turbulence - it was just a tad bumpy. We overflew only a few hundred feet over the summit due cloud at about 1500 above summit level - there is a handy 'get-out' if things get horrid; you can dive down into the valley to the north. But we had no such problems. Same goes if you get engine failure - glide down into a valley - there are some relatively flat areas by the rivers.
If you go during the week, it's esential to talk to Valley because the place is crawling with RAF Hawks, helos, and stuff.
Flight plan? I've never bothered (never occurred to me). Can't hurt to file one, I suppose.