Several board members have made their own solar gain devices, one of my neighbours has also tried this.
I can tell you that you will get heat gain from just about anything black, but noticeable heat gain is a different story.
The most efficient way is to increase surface area - you can do this by creating a manifold and having numerous SMALLER black pipes running to the collector manifold and back into your regular pool plumbing.
You will have to figure out how many taps you need on the manifold for the size tube you're working with, but the cross sectional area of all smaller tubes should add up to the cross sectional area of your current pool plumbing - otherwise your flow rate and back pressure can be severely impacted which can lead to serious problems elsewhere.
Personally, I thought that 300 bucks (canadian) was a fair price to pay when figuring what I'd have to spend in materials alone (never mind my time I'd have to invest)for a home brew system that would have 25% of the efficiency of the panel I bought.