I noticed the part about free - but I believe there can be an advantage to handing over cash on some occassions - this being one of them....
Firstly, and to get the unpleasantries done with, some of the world's best plans designers are giving up due to copyright theft and the distribution of unauthorised copies. This is a loss to all of us. There are many junk plans available for free, but we all lose out if the really good designers get fed up of having their designs stolen and sold/distributed without their consent.
Right, now that's over with, the advantage to the end user of buying plans is that almost always we are spending a few pounds/dollars/whatever into something which we know is going to fly. We are unlikely to waste balsa and expensive plywood cutting parts which later don't fit due to bad plans. Additionally, we can usually get high quality fibreglass engine cowlings and the like as well as good, clear, well fitting canopies for our models. Lastly, there is often the benefit of others (and even the designers themselves) to offer advice and support on a known product.
A set of good quality plans represents a miniscule proportion of the total cost of a scale model, so my choice would always be to buy plans from a reputable designer. At least I know that a model built from those plans will fly well
Of course, this doesn't apply to all subjects and there are some good free plans around. With something like a Thunderbolt though - why chance it? It's well covered by several good designers and all are proven to build well and fly well.
