I use the Pantone Huey pro, it does what it says on the tin.

It work very will in combined use with CS3 and the out of gamut warning and print previews, to gift a fairly accurate colour view but it is always too bright in comparison to the printed final.

Additionally if you working in Adobe RGB and then run gamut warnings then print to a sRGB print lab then there is no point anyway as the colour spaces differences mean you going to get clipped colours anyway.

Additional with the Pantone you get free colour palettes from Pantone to download and some articles which are really good if your going to be combining images into more art / digital publishing.

Hope this helps.

I am sure if i am technically wrong with the above someone will correct me.