Anyone has / use impact wrench?
Is it useful tool to have for working on cars? Or is it a must tool to have?
What have you got, if you do? How do you find it?
There are three basic types. Air powered - I did a big bit on my setup 3 weeks ago here in tech-talk. (entitled Compressors, rattle guns ----------) Reading it will give you some idea of how much gear you need to back it up, can get expensive, especially as you need a good compressor to get the best out of it, so I'd only go down that road if you are going to get deeply involved.
Then there's battery powered. Used to be they couldn't hold a candle to the air powered tools but they are getting really quite good these days. Quite expensive for a good make and, like with battery powered electric drills, if you only use them infrequently, you'll probably find the battery is flat just when you need it!
A real "outside the box" approach might be to consider a mains powered one. Of course you've got to have access to mains electricity which makes the battery jobs more handy for some and I wouldn't use one in the rain or standing in a puddle! but the overall cost is a bit cheaper than a good battery tool and it's always ready to go as long as you have a plug in point handy. The Clarke CEW 1000 would be my choice (available on line or from Machine Mart) MM's spares backup is quite good in my experience. The only issue with the mains powered ones are that they are not very adjustable. By which I mean, pull the trigger and it's full on GO! The battery and air tools are quite finely adjustable. Mind you I use these types of tool for removing fixings only as I like to "feel" how a bolt/nut is running up it's thread and tightening so use hand tools for tightening. "Full on GO" works just fine for undoing things - unless they are very small (m6 maybe?) as you might snap them.
One thing a really powerful "rattle" gun excels at is removing really tight fixings where you either can't, or find it very difficult to, stop something rotating. The crankshaft pulley bolt on an engine or perhaps the driveshaft hub nut on a front hub assembly.
Most things however you can do, albeit rather less quickly, with a nice big power bar - and maybe a length of scaffold pole - and the application of a bit of muscle and/or body weight! Don't forget you'll need to buy impact rated sockets to go with an impact tool. There is a very real risk, if you use ordinary sockets (especially cheaper ones) of them shattering with bits flying, at high speed, all over the place!
So I think, nice to have one but, if you're not doing a lot of jobs which make use of it, you're going to be able to do pretty much everything without one but it may take you just a little longer and make you sweat a little more! You can buy a lot of nice hand tools for what a good impact tool will cost! If you do go for one avoid the cheap bargain ones. They often make lots of impressive noises but don't actually apply a lot of force to the work piece!
Good luck with whatever you decide. Do let us know if you buy one and let us know how you get on with it if you do.
regards
Jock