I'm a roller guy, rollers with resistance, that is. Rollers without resistance are a tool, but it only has one blade. In the winter, I work on high cadence, OLP, and do some endurance work on the rollers. I don't do intervals in winter other than Tabata intervals, which work very well on rollers. I also lift weights in a gym, work the Stepmill, and take spin classes. Metabolic circuits are probably a good idea. I'll try them this winter. But non-bike training doesn't take the place of bike training. Even spin class isn't useful, other than as a fair aerobic workout in a fun setting. I like rollers because they're a hyper simulation of road riding. Even more effective than the road because of the lack of momentum in rims and rollers. I like very light rollers - plastic is fine. One can even stand on rollers or use aero bars once one gets the hang of them.