Mac OS X Lion really rocks. But if you depend on WiFi (e.g. you have a MacBook Air) BEWARE!
Lion has a bug imbedded in its fur somewhere that makes WiFi problematic. It will appear you have a connection, but you may not be able to access the Internet. There are many ways to temporarily resolve the problem (switching off WiFi and re-enabling sometimes works), but there is no overall work-around.
So if you have not already taken the plunge, we highly recommend NOT to upgrade till OS X 10.7.1 comes out, which hopefully will resolve the issue. Note there is no easy way to downgrade your OS back to Snow Leopard once Lion is installed.
(The problem does not impact Ethernet connections.)