As Jim notes, the Dual Field is well spoken of, as are a couple of others.
I was looking at a couple of different PI units when a Sand Shark came up used at a very good price. I figured if I did not like it, I could resell it and do no worse than staying even. That was about 1 1/2 years ago and I still have the Sand Shark.
I'd like to try a Dual Field, an Infinium and a Head Hunter PI. So far I haven't seen a deal that said to me I have to do it.
The Sand Shark does it's job. I will need a very good deal to change it out of the line up ... at least until all the kids are through college. Then the cash flow equation should change a bit and I may be able to go with more toys ... oh, sorry ... I mean tools.
Cheers,
tvr