That's a good question. I've been wanting to get one. If its not accurate its worthless.
Also if it doesn't lasts it doesn't help. I was in BP and saw some good ones going for $50 and up. Since I knew nothing about them
I passed them by.
For the money these are worth a look.
http://www.ebay.com/sch/i.html?_nkw=...cale&_osacat=0