I'm giving the Wacom Intuos5 a go at the moment and the first few hours of using it I've been blown away at how easy it makes retouching and editing in both Lightroom and Photoshop. It only took a matter of minutes to get used to the hand-eye coordination required and then I found that using it alongside the keyboard of my Macbook Pro, it made editing more accurate, easier and quicker. I've got the Small Intous5 Touch here and I wouldn't want any bigger to be honest, the surface area is perfect.
I'm not really into my portrait retouching but I gave it a go with the Wacom and it actually made it enjoyable! The pen is very responsive and you can turn on pressure sensitivity so that you don't have to keep adjusting brush sizes. Here's before and after pics from a quick 5 minute retouch: