The type and quality of activated carbon does makes a big difference. Diego214 I don't think those are worth the money at all. If you are handy you could build a better unit that fits your needs and save a ton of money. I personally don't like those units because of the floor space they take up. I like Can filters because they work great hanging from the ceiling to save me all the floor space possible. They are easy to rebuild just drill out the rivets on the top of the filter , remove the top, empty out the old carbon, when replacing with new carbon make sure that the center tub is inline or it'll be hell to get the top back on, tap/shake the unit to get all the carbon you can in it, put the top back on with some self tapper screws and you're done. If you have any old Can filters you can easily replace your carbon with good carbon on the cheap. I found a local source for activated coconut carbon, but am sure people will ship it to wherever you need. Just remember when you do replace the carbon it's fuckin' messy as hell so take off the church clothes first. Here's a couple of tips:
It's better to pull the air through whatever type of carbon you are using a slower rate vs. fast. You want more surface area at a slower rate of air passing through the carbon to do the best job possible. Basically put a speed control on your fan if you have too big of fan and make sure if you are using a Can filter is has room around it.
Keep your humidity in check because if it gets to high the carbon will not be as efficient
Carbon doesn't last forever! You should replace it every 6 or so months depending on your usage and etc.
I like to buy the biggest filters out there and usually run one of them 24/7 venting back into the room. They are also great for when it comes time to trim. I took a old Can filter and slapped on a inline fan and take it into the trim room during the trim and drying process and this cuts out so much of the wonderful smell