Since moving to ActiveHDL, Ive found a load of bugs, but these are mostly on some more eosoteric VHDL 2008 stuff. But they were quite serious (simulator not mapping unconstrained record access types to system memory properly - so populating my records with data from adjacent fields/rubbish, and then eventually crashing).
Currently, active HDL doesnt like having constant records with Null arrays = vsim crash
This is the kind of stuff I had working in Modelsim previously.
ActiveHDL really likes you to use their tool as a full development environment - which is very annoying if you're using to driving everything from modelsim's command line. ActiveHDL inisists you have a "workspace" open to function when in the gui. Luckily, you can run it in batchmode and view the waveform without having a workspace open.
ActiveHDL is stricter to the VHDL LRM than modelsim. I had some code I ported that worked in modelsim ended up failing in ActiveHDL (for correct reasons). They werent even warned in Modelsim, so I suspect it is actually a bug in MS and a SV carry over.
But at the end of the day, ActiveHDL is much cheaper.
Both are good at dealing with support requests, but Aldec are pretty quiet - they tend to just pick up the case without any response other than the auto-generated reply. MS would at least give me some sort of "thanks" reply.
- - - Updated - - -
As for performace - I cannot say. Without running the same design through both I cant say
- - - Updated - - -
And finally - ActiveHDL is a native application. Modelsim GUI runs inside a Tcl.TK gui - hence rather slow and has always been a problem for it. I think the newer versions are moving away from tcl.tk