I have both the Bissell ProHeat Upright and the Bissell SpotBot. I prefer to use a carpet (or fabric) cleaner in a spray bottle, spray, agitate, dwell, then use the extractor with just hot water to rinse/extract the soap and dirt. I've used a number of different carpet cleaners. For fabrics (like seats), I use our liquid laundry soap (Era) mixed with water....again, spray lightly, let dwell and then extract with hot water.
I don't like using soap in the machine because unless you empty, refill with just water and do it all again, you just don't get the soap out. It just makes more sense to me to clean, then extract with fresh. clean, hot water. Frankly, sometimes for light duty, a hot water extraction alone is all that's need to freshen the area's....hmmm might try adding a little Fabreze to the hot water for some extra freshness.