I recently got a Pneumatic Antenna Launcher from Alan Biocca Engineering, the CSV17. Very well made, fires weighted tennis balls, and pumps up with bicycle pump.
Every other launcher I've seen fire lead weights which are asking for trouble, unless you're going over a tree with nothing of consequence behind it (e.g. an empty field).
I got a line over a Fir tree branch at 60ft on the first try with 50 psi (the 90 psi max will go 150+ ft), and with-in a couple hours in the rain I had my OCFD up where it clears the other trees around. At the previous height of 40ft I was always getting snagged in the Maple trees, and couldn't get one leg fully clear of one.
The OCFD has come to life on 80m and 40m, signals are noticeably stronger, and it's uncovered a layer of weaker signals previously lost in the noise. I've started to hear more DX on 20m too. On 6m the local repeaters are much stronger which suggests it might be useful here too.
The antenna launcher has already paid for it self in ease of getting a lines accurately over trees, and improved antenna performance as a result!