Actually, once you get a feel for the layout of Convoy, it's easy to play any class on that map. As for snipers, yeah it's a pretty open map but that means that the sniper positions are pretty open as well and it always turns into a sniper war. The snipers take turns shooting each other while I run around stabbing everybody.
For my opinion. Convoy is my favorite map in all of TF2. In fact, I joined eGO because of our Convoy server. I was looking for Convoy servers to play on one day when I stumbled into ours. I've been coming back ever since. If you take a look at the logs on that server you'll see that we have the most consistent players of all eGO servers. Players that play that server come back day after day, and the fact that most of us seem to be older players might explain why the server is kinda bare in the mornings and then fills up when we get off work and stays full all night.
The fact is, Convoy is almost an eGO exclusive. There aren't a lot of servers out there running the map any more and the ones that do are so poorly admined that I can't stand to play on them. Our Convoy server is well admined and can guarantee a good time for all players, just ask anyone that enjoys the Convoy map, they'll tell you that our Convoy server is worth keeping (and, thankfully, the counsel agrees).