If you ever did want to change filters, it shouldn't be a problem to just cut out the plastic outer and transfer the rest to a new filter. I do use the cartridges that came with it and no problems yet.
One of the problems I can see is that the water theoretically only needs to pass through one cartridge to exit the filter, but having said that, while there are different cartridges you can get I don't personally think there's much difference (there's one I've got that claims to reduce ammonia, but I think that's a load of rubbish as I've not seen any different in my cycle to what you'd expect so far). This can be a problem if you wanted to get a bit more involved with different types of filter media (it forces water from back to front rather than bottom to top), but it does the job. There is a little bit of space in the back if you wanted to put some extra stuff in there (mine is currently stuffed with some mature media), and you could put some carbon in there for absorbing meds and the like later if you needed to.
I see no problem with putting your own sponges or whatever inside the filter, as long as you rig it so that water HAS to pass through it to exit the filter.
I think what I'm trying to get at is it's a good, basic option. Probably not as advanced as some people would prefer as you can't go nuts with different filter media, but I've been runnning it for my cycle and it's not doing bad so far. I did a water change and removed my gravel and put sand in last night. Despite washing the sand out, my water became so cloudy when I put it in you could only just see the heater light. Now, less than 24 hours later, it's almost completely cleared.
I also like that you have easy access to it for maintenance and whatever else you might want to do to a filter.