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Another vote for Concept II. New ones cost about a grand, used ones hold their value, but it’s worth it IMHO.
Every piece of indoor exercise equipment I’ve ever had was boring as hell. I’ve tried TVs to watch and headphones hanging from the ceiling (to keep the cord out of the way). I found headphones and music better for getting out of your head for long aerobic workouts.
Don’t get one if you have back trouble. It will only make it worse.