All of the functionality of The GIMP remains the same with the bonus of a much easier to use Photoshop style interface. One of the biggest complaints of people switching from Photoshop to The GIMP is the unintuitive GUI of it which is why the developer designed GIMPshop.
GIMPshop features menu layouts, dialogs and naming conventions that all Adobe Photoshop users will be familiar with. However, one of the main problems Windows users will experience with GIMPshop is stability. GIMPshop is an open source on-going project and so it is not a fully polished product. The GIMPshop forum isn't particularly active either so there seems little chance of getting any help on this. There are also certain features missing from GIMPshop such as recording actions that are only available in Photoshop.
Of course, GIMPshop isn't a replacement for Photoshop but it's probably the closest thing you'll ever get for free.