The Twin 1 is available with a retractable gear or as a trainer version (fixed gear, no water ballast). The RG version suffers from too large geardoors which are torn off easily in outlandings. Although the gear is fixed, the trainer version also has the box for retracting the gear in the fuselage.

The Twin 1 has slightly better performance than the II (which is already better than the ASK21) and a higher payload. But it's heavy on the controls and the backseat is quite uncomfortable. There's also a noteable lack of control harmony compared to other glass gliders. These are the main reason for their low price. We sold it after a few years and nobody really missed it. In my view, if you get a Twin II with reasonable payload, it's a much better choice.

As for the single-seat Grob gliders, there's also a technical note on applying turbulator tapes on Twin 1 and 2 wings. It improves handling in thermals, but doesn't help with the lack of harmony and stiff controls on the Twin 1.