Anyone using one and what do you think about them and what units do you recommend.

I have used this type:

http://www.sportsmansguide.com/net/cb/cb.aspx?a=295740

They are not necesarry, but handy at times. In my opinion they ensure that you get your first shot on paper. You will not be truely accurate without firing a single shot or anything.

I sighted in my iron sights on my AR pistol between 25-50 yds without firing a shot and it's reasonably accurate. I'm sure I couldn't shoot apples out of a tree at 100 yds or anything.

All boresighters just get you on paper.

The laser bore sight device, is nice to have, but, not necessary. Spend that money on ammo.
Fix your upper in a padded vise/rifle rest, whatever. Align the bore on an object 100 yards away. Set sights/scope/red dot etc centered on that same object. Now go to the range and set your target at 25 yrds. Fire three rounds holding same POA, no matter where the bullets go. ( they will be on paper) You should have a nice tight group somewhere on the paper. Now secure the rifle, aiming at the original POA. Without moving the rifle, adjust the scope/red dot to look at the little group you just shot. Fire one round. Your POA and POI will be the same spot.  
  You just zeroed your rifle without a boresight device and with only 4 rounds. Now move out to 100 yrds and fine tune it. (I like 2" high at 100 yrds.)

You'll remember this next time you're at the range, and you see Billy Bob and Billy Joe Bob, blasting away at 100 yards with a new scope, and burning up a bunch of ammo trying to get her on target.

I had the Sightmark one similar to the picture above.

It broke somewhere between Academy and the range. If you don't have a boresighter just bring several large targets you can string together to increase your chances of hitting paper without a lot of hassle.

Thanks everyone for your input.