CADWorx Plant rocks for small-to-midsize projects. Very friendly user group and tech support as well. The "Pro" version comes with ISOGEN, while not ideal its the best I've come across. Models are portable, no object enablers required to view them (!), plus NavisWorks has native support for CADWorx data.
SmartPlant is excellent for truly large projects, but requires a lot of in-house support. It has a lot more tools to enforce consistency and do mass part manipulation and checking.
Stay away from AutoDesk Plant 3D. Its only had one release and isn't suitable for production use yet (unless you want to fight both the piping design and the software simultaneously).