I have done been on two rather long trips in Europe alone, and one with a friend from work.
If you plan to go with a mate, make sure your bikes have roughly the same tank range, and more important that the mate can keep up with you/you can keep up with him/her without being stressed. It's no fun for the slowest of you if you have to ride in a way you don't feel comortable to keep up with the faster one. I quickly got anoyed by having to stop for fuel/food/water a few km's after the last stop and waiting for him to catch up..
I'f you are a guy that has no problems with getting to know new people, I would recomend going alone. Evenings can get boring (but you are usually to tired to be bored after a long day in leathers and +30C), but the riding is best alone
I used a tent on my first Euro-trip from Oslo(Norway) to Faro(Portugal) and will not do that again. I have no problems with camping, but all the extra gear you have to bring is a real hassle. The only exception would be if you are staying at one place for a longer period. If you plan your trip either before or after August, finding cheap rooms/Bed&breakfast is not a problem. Most people are nice to nice bikers, and will help you find a safe place to lock up your bike for the night.
Things to think of concerning the bike/riding:
-Use good gear/leathers even when it's very hot!
-Inspect tires on each stop if going high speed on autobahn.
-Lube chain before you lunch when chain is hot, let it dry while you eat/drink.
-Drink LOTS of water.
-Check "dead angle" before making lane changes on autobahn.
-Don't use a tankbag if you are planning to use autobahns like they were meant to be used (a fast but boring after a while way of getting from A-B)
-Obey posted speed limits on the sections of the autobahn that have limits!
-Buy a good map of the area, to find the best roads.
-Do as many mountain passes as you can.
-Avoid the long tunnels.
-Look out for diesel on tight turns right after tank stops.
-Bring both a clear and a tinted visor for your helmet and something to clean them with.
-Make sure fuel tank is full before siesta time if going on minor roads. Some gas stations close for siesta.
-Visit the "Nordschleife"/Nürburgring if you are in the area!
-Bring a camera, and use it a lot!
Nice places : Nürburgring, The Alps (all of it), Lake Como in Italy!
I'm sure there are lot's of other great tips, but these were some of mine
Have a great trip!
