The first thing to remember is that WF is not a BPM tool - but rather a framework that can be leveraged to build a reliable and flexible set of business flows. If you are looking for a solution that provides you with a lot of built-in functionality where you won't have to do any coding but rather just put shapes on a sheet - WF is not your tool.
However, I would recommend WF highly. First, the performance in WF 4.0 is very good. Second, it's distributable across a farm of web servers. Third, it's flexibility gives you the power you will need down the road to reach those edge cases.
Finally, because you're looking to integrate with ASP.NET I'm going to say that WF is about the only feasible choice. I'm not saying that other options couldn't work, I'm just saying that it integrates well.
Another thing I want you to do is take a look at an answer I made recently about why you should or shouldn't use Windows Workflow Foundation - hopefully it's of value for you.