I bought Partition Magic a few months ago and have used it several times for setting up and for re-sizing partitions. I've found it both effective and simple to use (not being an expert, that's important!).
I don't see why it can't be used for re-sizing partitions set up by other systems; after all, it's only handling standard data to do it.
As far as cost goes, I got a copy from a firm called Worthy Deal (in Cambridge as far as I can remember) for about £26.50 including postage and packing which seemed very reasonable.
GG