Done both, each has pros and cons, but the more people the more frustrated i seem to get as each person wants to do things differently stop at different times, go at different speeds, will/wont use motorways, etc etc.
As for planning, going on my own down through France, Italy to Malta then back through Italy, Austria, Germany, but as I'm going peak time, last week July first week August, I'm booking most of my hotels. If I was going at other times I'd probably not book and trust to luck.
One Last comment, just fu*"ing do it you wont regret doing it either way but if on your own take lots of photos to piss your mates off when you get home