First, if you've never steam cleaned your carpets and think that vacuuming is enough - think again!! I have been using steam cleaners on my carpets for many years - in many different locations. It is AMAZING how much dirt, grime and filthy a steam cleaner will get up from a carpet that you have just vacuumed. Vacuuming only brings up loose dirt. All of the remaining odor-causing, allergen-triggering gunk that is even remotely "stuck" to the carpet fibers is still down there. You could vacuum diligently every day and a steam clean will bring up water that is as black as pitch. If you haven't steam cleaned yet, you definitely need to.

Now, that being said, should you go with the Bissell 9400? We've used many different steam cleaners in the past, so we have a fair amount of experience with the pros and cons of different unit types. First, the size. Many steam cleaning units are VERY bulky and unwieldy and hard to move from location to location. Yes, the Bissell is heavier than your average vacuum cleaner - but it is still liftable. I can get it up and down stairs without needing help. If you're on the less strong side, you'll probably want to move it without its tanks, and then bring up the water separately. That's fine - you should see what some of the "bulky" steam cleaners are like. This is about as good as you're going to get, and it's well worth the effort.
On the features side, this is great. You have both the main unit which you use like a push vacuum cleaner, as well as a tools connector for doing stairs and sofas and such. I highly recommend doing floors every 3 months or so. You will be amazed how much household debris the average floor will collect in those 3 months, just from feet and pets and general dust in the air. Just look at the color of the water that comes up after the cleaning.
A great feature of this unit is that you can select the level of cleaning you want. If you have lots of pets you can go for the heavy detergent. If you have just a few humans, you can go with the light level. This helps you optimize your detergent usage for what you need.
You definitely need to move SLOWLY with this unit. It is squirting water down into your carpet fibers, and then on the next pass it is sucking that water back up again. The more slowly you go, the more thoroughly it does its job well. Don't race through this. I recommend putting on some music (fairly loud) and looking at this as a relaxation time for you. If you have noise canceling headphones and an ipod, that works well too. If you have "books on tape" to listen to on your ipod, you might find yourself doing some extra passes with the steam cleaner because you are having fun listening to the story

So you get a workout for you (in the sense of walking around), a clean house, fewer allergies, and you enjoy a story. What else could you ask for?
The unit has a built in heater. This won't heat up ice cold water, but it does ensure your hot water stays hot while you slowly walk around. So it is definitely a great feature.
Now, the downside. Unlike many units, this has a "combined" water container which has both the fresh water and used water in next-to-each-other bladders. This means you can put in far less water than in most units, meaning frequent refills. I'm not sure at all why they did this. It makes it hard to fill, a pain to empty and clean out, and means you have to keep going back to the sink to get more water. It doesn't make a lot of sense.
Plus, the way the system is set up, the flap to "end suction" is very sensitive. If you fill up the fresh water bladder to the fill line, it cause the flap to close!! Ironically I saw a review where someone thought the closed flap was necessary for increased suction. No!! The closed flap means it is NOT having suction any more. It is only supposed to close when the used waste water gets high enough to trigger it to close. If it prematurely closes it means you have lost suction and all you are doing is filling your carpets with water. You need that flap OPEN for the unit to work properly. This is a royal pain. Make sure you only partially fill the fresh water bladder so that the flap does not trigger to close. Since the bladders are so small to begin with, this causes you to take even more time than you should in returning to the sink for refills.
It is amazing how much hair the unit will pull up from a "freshly vacuumed carpet". That being said, the Bissell just randomly drops hairballs all around as it goes, which means you have to then stop and pick them up lest you get them back into the innards of the Bissell. It would be much better if it had some sort of a lint trap which collected them for you to easily deal with later on.
Still, I love this unit. I don't think those negatives are enough to knock the review down from 5 stars. You deal with the lesser sized bladders. You pick up the hairballs. This unit does an *awesome* job at cleaning carpets. Again, anybody who doesn't steam clean, take the challenge. Give it a try on your "clean" carpets. See just how much dirt a steam cleaner will suck out of them - and notice how much better your allergies and health feels once you are breathing clean air again.
Highly recommended.