We're going whole-hog (for those folks not from the South, this means that we are ardently pursuing a course of action - it just takes a lot less typing to say it) into AutoCAD P&ID, and really gaining the benefits over the old-school lisp and vanilla AutoCAD methods we used before.
But like any other Autodesk application, you've got to follow some rules. Here's a couple of tips that will help with some of the errors.
Off-page connectors - Autodesk released a fix for these not too long ago, but there's a better way to screw this up. If your pipe line segments between the connected drawings are not the same type - for example, you used the Primary Line Segment in one drawing, and then used Secondary Line Segment in the other drawing, where you wanted to make a connection - you won't get the tag properly labeled. We did this with several lines, just not paying attention. Off page connectors require the same line segment type to properly connect, so check this in both drawings before you implode.
The other good one relates to Validation. We've been using the Validate tools on P&ID objects, but we also place all of our geometry in paper space - since we're not working at scale. If you configure the Validate tools to check for Base AutoCAD objects, you might not get anything to show up as an error - even if you've added all kinds of cheatin' shortcuts.
The Base AutoCAD Objects check only looks at model space - so if you are importing old P&ID's to convert to intelligent objects, do your work in Model space, and then check. You'll find all of your random linework and be able to edit them right out of your shiny new diagram.
I've got more, but fixing all this has made me tired...time for a nap...
Later - David B.