Besides shedding the most weight of your car possible in one shot and helping weight transfer and suspension geometry, it also gains you more space or room to work with. For example I can pretty much pull my oil pan without having to jack the motor up at all.
The other thing is full adjustability of your front end.
with the engine out of the car the swap is fairly simple.. although i've seen it done with a jack, some jack stands with an engine still in there.. but that didn't look like fun if you don't have a garage
I used UPR in mine and i love their setups. a lot of people will argue their quality control.. i've never had a problem with quality in either of my stangs, and their customer service was AWESOME.. those guys will talk to you for a half an hour while you are laying under your car scratching your head just to walk you through any problems
UPR ftw. i was at Budds Creek ( dragstrip ) a couple of weeks ago for Ford Fever and UPR was a vendor there and they were having a show special : a complete k member with coil overs for $599 ,. i was a little bummed since i paid $ 699 for mine from them but even at that price i consider it a deal.
Install isn't as hard as people will make it out to be if you have some experience with working on your car. You can rent or barrow an engine hoist, unbolt the motor mounts and raise the engine up inch or so and go at it.