I just bought a Blueant S4 for my Dad after reviewing a number of different devices. It's particularly good in concert with an Android based device apparently, and there are rumours about new apps for the iPhone 4 making it more compatible. You should be able to pick this up for about $80.
I also looked at the Jabra Freeway as a hands free option, but read too many reviews from people disappointed with its performance (including a Youtube review where it repeatedly failed to recognize what the guy was saying).
If you're not worried about hands free (as in no buttons to push), your options open up significantly and the price drops too. The most tried and true of the bunch appears to be the Motorola Motorokr, which is basic but very reliable according to most reviews. The Jabra Cruiser 2 also seems to do pretty well.
Have a look
here, and if you're considering hands free, check out youtube to see the things in action before you decide.