I've used at least one cordless impact almost daily for the last 6 years. I 've owned the makita 9.6v 3/8 drive unit for 6 years . it has performed flawlessly and still has both original batteries working great. I use it daily building engines and working on race karts, several racing buddies have the newer 14.4v modles and they are every bit as nice with a little more power. On a typical race day or weekend, the 9.6 will get enough run time to change batteries once, litterally running off and on 100's of times 1/4" to 3/8" sized fasteners. My 9.6 model will tighten up 1/2-13 (3/4 sized) nuts on anchor bolts and pull them with washer clean through a fresh treated 2X4.
two years ago, I bought the 19?v harbour freight 3/8 drive impact on sale for 29.99. it works well but is larger and heavier. the socket detent isn't very good and older sockets tend to fall off. biggest drawn=back to the HF unit is that the battery charger isn't automatic, supposeddly can only leave batt on charge for 5 hours max....I just plugged it into a cheap timer and set it to 5 hours.
I also have two Dewalt 18v unit, 1/2" drive at work. they are beasts and come in two sizes. I have the largest one and the one under it. They are great for Substation work, but I would rather have the smaller makita for around the house stuff. the 18v dewalts will wring off 3/8" threaded fasteners if you are not carefull.
Home Depot did have a great deal on a Makita combo pack with impact, drill and light, 14.4v for like $229 several weeks back. I paid $185 for just the 9.6v impact 6 years ago.
For most use, I'd stick with the 3/8" drive units, the 1/2" drive ones are usually bigger abd overkill for most projects. I don't really care for the 1/4" hex drive quick couplers, but there are adapters for sq drive.
The ridgid and ryobi and hitachi units all look good also, If I was starting from scratch, I'd have to look real hard at the ridgid with the lifetime warranty.