To greatly simplify, PCI Express claims huge bandwidth reserves, easier trace routing, and hot-plug capability. It delivers on all of those. Whether or not PCI Express devices need bandwidth and hot-plug is another question. It's clear, though, that PCI Express will save on design and manufacturing costs in the long term, and offer plenty of bandwidth for many years of future growth.
As for the other half of your question, there haven't been any major problems on the desktop, though Intel's server/workstation chipset codenamed Lindenhurst was/is having PCI Express problems. For all practical purposes, no device right now needs PCI Express or takes advantage solely from the interface upgrade.
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