I'm considering to use PlantUML. One thing I have stumbled across is the strange use of diamond shapes in the activity diagram. I don't know if it is really UML compliant to write a question in the diamond shape -- at least I don't know of any other tool that does it. And I certainly don't like it.
I found, however, that I can get the commonly used diagrams by using empty conditions and labels with square brackets. But there is one exception:
while () is ([cond met])
doesn't work. I have to put at least one space between the braces after "while" (which ruins the diamond shape).
As it is possible to use an empty conditions in "if () then ([cond met])" and "repeat while () is ([cond met])" I think this is a bug.