If you're looking for a challenging kit and you're into bipes the GP Super Skybolt's great... took me 3-4 months to complete. I'd previously only built the GP PT40 kit, and done a few crash rebuilds, and didn't find anything too challenging in the Skybolt as the instructions are well written with good illustrations. From looking at wzak29's build thread on the GP Super Sportster it seems that they've used very similar designs in both the Sportser and Skybolt, with the addition of an upper wing.