We did the big car trip in 2010, picked up the car on the way out of Paris and dropped it back at Calais 50 days and about 10,000 klms later. We basically did the whole of Western Europe. Returned to Calais to avoid the one way cost plus the ferry is a lovely way to arrive in England both from seeing the white cliffs at Dover then a pretty pleasant train ride into London but really it would not have been a significant cost. All in all, I wouldn't hesitate to do it again though there were both advantages and disadvantages
Advantages:
1. You can organise it all from Australia, we used Globalcars and it was a very simple process. They were very good to deal with
2. A brand new car, insurance, GPS, phone support/assistance etc all covered with no more to pay (well other than your running costs, tolls and parking)
3. Lease costs, petrol, tolls, parking probably cost us about 10K all up. There was my wife, 13 year old daughter and myself and really, I don't think we would have saved any money using the train or flying
4. Flexibility was fantastic for us, we had a fairly loose itinery so if we wanted to take a detour on the way, change our plans or stay somewhere a bit longer there was no impediment to doing so. We ended up doing so many things that we would never have thought of if we hadn't been in the car and in a way many of these things were what really made the holiday one of our best ever, even the numerous times we got lost
5. Great way to store luggage as especially for those one and two night stays, we left the majority of our luggage in the boot and only got out what we needed. We used a sedan so luggage not visible but don't underestimate how much easier it can make it especially when you are tired after a long day
6. Did some unbelievable drives, ie. the Almalfi Coast, both exhilarating and scary at the same time but a long time item on my bucket list, driving through the Alps and Pyrannese, being able to drive to Kitzbuhel and then being able to ski the downhill (very slowly mind you), something else on my bucket list and so many other things
7. Traffic wasn't an issue, we tended to arrive into major towns in the afternoon and were generally going against the traffic
8. Great in country areas and small towns where a car is an essential, Tuscany for example there's a lot to see and a lot of driving required, a lot of fun to with the winding roads and cobblestoned villages
9. It's not like Australia, places aren't that far apart so there's not need to do 10 hours a day to get from A to B. So you don't really feel that you are missing out on things because of the driving, it's part of the holiday rather than a chore
Disadvantages:
1. Capital cities and other major towns - often had no use for the car and also these are places where you tend to stay longer. Parking was expensive 40 to 50 Euro a day for a car not being used though as noted above, I think my overall cost comparable to any other option. We did tend to stay in the middle of these cities, no doubt parking much cheaper away from the city centre.
2. Certainly some stress, driving on the other side of the road into areas you have never been to does require concentration, there were times I that I wished I wasn't driving. Would have helped if my wife did some of the driving but not to be so it was me driving every last inch of the way. Still I have driven in India and China and Europe is a breeze compared to those countries
3. You need to be aware of specific requirements/rules in some countries. I knew I had to get a pass for Austria but didn't know about Switzerland. Nothing happened but apparently I could have received quite a big fine if I had been pulled up in the five days we drove through there
It's been a while so there's probably lots of other points I have forgotten to mention but hopefully I covered off the main ones
Overall, a huge thumbs up from me. The flexibility and ability to see so much more just blows the other options out of the water for me. My plan going forward is to keep driving when I can as I know there will come a time when I can't do it anymore. The I'll try the trains and flying!!!