What about the fact that I had driven it so far while it was hot?

I babied it by driving slow & pulling over to cooldown on the big drive back home, but still... Even in town, I drove to the garage etc. However, the resevoir has never boiled dry and it never overheated again, althought the resevoir was boiling when I got to my destination.
Based on this, people have been telling me that my head gasket is probably blown & I should consider replacing my engine.

Is this the case or are these people being overly pessimistic in their diagnosis?