I wished I had satnav last summer when tryin to get to & from Jablines camp site near Disney. Got lost both ways because the road signing is so bad. Ended up going right round Paris the wrong way coming back. I bought a cheap PDA and GPS on eBay then put TomTom Navigater on it. What a difference, it takes us where we want to go by the best route.
It doesn't have the new roads off the N136 near Rennes but if you follow the signs, it soon puts you right again.
It's specially good in cities. On our annual visit, we used to drive into Southampton from the airport on the motorways. The satnav takies us by the route the taxis use.
We live in the Channel Islands so can only use it on holiday, but still think it's worth the expense and I would recommend them to everyone.
As for the type, I think they're all good so try before you buy if you can.
There's loads of info & advice here http://www.pocketgpsworld.com/ especiaally on the forums.