If you are doing the bags the quality of the ride will depend on the setup. I have seen a lot of rides with poorly designed system. They install 4 valves only in order to save money and it results in the crappy ride as you have 1 fill and 1 dump valve for both front wheels and same for back wheels. 2 wheels on the same valve is not a good idea as it allows air to travel from one wheel to the other every time you hit a bump or make a slightest turn. For example, if you are making a left turn, the weight shifts to the right side and pushes the air out of right bag into the left, causing increased body roll and crappy ride. Changing lanes on the freeway feels like the car is floating like a boat.
I hope this makes sense.
Another common mistake is not running shocks in the front.
When done right air bag ride is very comfortable and makes the car ride smooth. And no you don't need to spend $4-5k to have a good setup that will last for years.
To bag the car correctly you need 2 valves per wheel (1 fill and 1 dump), this isolates each wheel and dramatically improves the ride quality. Each bag is independent this way.
On my 67 I have a simple setup that I put together my self with quality components. 2 compressors, 2 5g tanks, quality bags (Universal Air), mounting brackets, weld in shock re-locators, 1/2 lines, 2 dual gauges to monitor all 4 bags and 2 switches (1 front 1 back).
My complete setup was little over $1000 and I installed it my self. It's been on the car trouble free for last 3-4 years. I definitely don't recommend buying kits on ebay or similar.
The ride quality is amazing, not as performance oriented as coil-overs on my Chevelle, but I don't take the Impala to the track either. It's more for cruising around with family and for that purpose bags are doing an amazing job, much better than (lowering) springs.