I have guaged my players strengths and weaknesses in my Saturday campaign. I have a long history of throwing extremely challenging encounters.

An example: A phasmadaemon created a completely illusionary fortress and filled it with derghodaemon shock troops.

The dhergodaemons weren't too big a deal except for the fact they could get full round attacks through walls that don't exist in a long running combat. And the phasmadaemon could strike with spells thrpughout the combat. It took a lot of effort for the party to survive it and the giddiness when they finally took down the phasmadaemon was palpable.

That's why many of us play... The struggle to defeat enemies that requires serious thought and effort, the final great blow that finishes that horrible enemy that has frustrated the party for so long, the moment your character stands in triumph and reaps the impressive rewards.