The TK3 was my choice of semi-rectangular bags. I like to be able to move inside the bag and wanted the versatility of being able to zip the bag partially or completely open for ventilation. I've slept in temps plus or minus freezing this winter and stayed comfortable. As pointed out, the bag is well made and comes with useful accessory storage.
It would be perfect for me if it had slightly more room across the shoulders--it's a bit restrictive--and if it had some method of holding the lighter-insulated bottom of the bag to the pad -- a sleeve like my Big Agnes summer bag or attachment points for straps to hold the bag and pad together. If the bag turns with you, rather than you inside it, your back is 20% less insulated. There are hanging loops on one side for airing the bag, so I may try to sew some on the opposite side for strapping the bag and pad together. With slightly more shoulder room and a bag/pad system approach, the bag would be wonderful. Listening Sea to Summit folks?