There are a couple of downsides to Terragen. First of all, other than a downloadable manual and their website, there's no support available for the free version. Basically, that means that unless you're gifted at teaching yourself graphics software, or dedicated enough to keep at it until you get it right, you might want to pass this one by. Realistically, however, you can't expect unlimited support for a free program. For other purposes, you can purchase the software on line through CommUnique.
Further details appear on the software's website.