I use TaskJuggler - it's more powerful than other OSS software I've tried, but it's very complicated. It's the only software I found that could do what I needed, but I still consider it to be "best of a bad bunch". I too wouldn't mind paying, if I could be sure the software would do everything I needed.
The big flaw with planner seems to be around resource leveling/allocation but I admit I haven't looked deeply into it. Planner would be my second choice.