I've got one got it when I thought I'd have to transition from 3ds max and just to see what they are like (cant grumble at £30)
I'll try and give you an unbiased rundown:

pros
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- Gives you a flyaround/flythrough ability so that you can easily and quickly navigate around with your left hand while using the mouse for other things
- can configure the two buttons to do just about any action you need (I've got one set for the generic move/rotate/scale tool which is handy)
- gives you a universal control system across multiple 3D apps or others such as in photoshop. great for quick transitions.
- once you get used to it it seems a bit wierd going back to alt - moving method
- lots of configurable options e.g, you can use the push up/pull down for up/down or for forward/backwards and set things up just how you like with the software app
- sdk so you can reconfigure/reprogram it for custom use such as in games or other apps.
- great freedom of control. pull up/push down, rotate left/right, tilt left/right, tilt forward backwards, forwards/backwards, left/right actions

cons
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- could realistically do with more buttons so you dont have to touch the keyboard
- sensitivity can be a problem. I've got it quite sensitive so that I can quickly pan around at a distance but when up close it seems a bit too sensitive and flies around.
- takes a while to get used to the sensitivity of the pull up/push down action in combination with forward and backwards
- occasionally if I'm zoomed really far out it is quicker to alt- drag to zoom in real fast. Again this is probably just to do with the sensitivity and configuring the options that you prefer. Alternatively you could configure one of the buttons to be alt and this might help.
- you sometimes still end up letting it go to do certain keyboard actions

Overall I'd say its a nice tool that you can quickly get to grips with and helps to navigate around nicely. I could get used to not having it but would feel a little like it was a step backwards.
At £30 for the personal licence its not going to break the bank if you dont get on with it

Edit: steve - it does indeed mean you can select a vert/edge/face and rotate around it at will without deselecting or anything else. I find it particularly useful in really tight geometry areas where for some reason you cant see the little white square to select the face. I just zoom in and fly around and select it before flying back out

Edit Edit: The Architect - sounds like you're talking about a microscribe. They have one at uni over here. You basically set the mode to nurbs curves and stick a real object on a grid (calibrated to the pen). Then you put the end of the pen at the points that you want the control vertex to be and press its button. You keep doing this till you have many nurbs curves that you can then use as a basis to extrude your model from. They work pretty decently but for everyday people are far too expensive to even consider and to be honest I'd see it as a better exercise to model it the usual way from observation

Si