I had read that these filters were of a very high quality, but that their prices were generally higher than other HOB power filters. However, they became discontinued at my local Petsmart so their prices were absolutely stunning and warranted a purchase.
The filter seems to be well built and unlike my Tetra and Marineland HOB filters, I've never had a single problem with this filter's impeller getting stuck. It also runs dead silent and has an adjustable flow rate.
The filter has two "cartridges": the first is a corrugated thin foam material that is infused with activated carbon. The second is a biomedia sponge material. I personally don't use carbon in any of my tanks, so I used a sharp knife to cut out the corrugated carbon foam material from the first cartridge. I then put the remaining plastic frame into a mesh filter media bag and stuck in a bit of filter floss.
After a few days running on the tank (which was already established, stocked and was running a Penguin filter in addition) the flow rate through the Eheim filter was dramatically reduced. I wasn't too worried as this was really only a secondary filter, but if this were the lone filter on the tank I would be disappointed. I attribute this to the fact that the width of the area for the cartridges is *much* narrower than with other power filters, which means that accumulating debris slows the flow rate quicker on this filter than on others.
Which brings me to the biggest problem I've had, which actually prompted me to write this review: I've been out of town for about five days, leaving my aquarium care in the hands of my brother. Right before I left, I put a couple of big scoops of water lettuce in the tank. During the five days, the Eheim filter actually sucked up enough water lettuce to significantly block off water flow through the cartridges. As a result, the filter was pulling in more water than it could dump back out, and water slowly poured out of the not-quite-watertight lid. 3-4 gallons of water were effectively pumped out of a 29g aquarium over the course of 5 days as a result. I think this is due, again, to the narrow width of the cartridges and the resulting low flow out of the filter.
In conclusion, this filter works very well assuming that you keep an eye on its flow rate and give the media a good swish in old tank water on a regular basis. Unlike a Tetra or Marineland filter, if the cartridge gets clogged, water won't simply flow over the top of the cartridge and out of the filter, but will rather spill out of the top. I wouldn't buy another at full price, but would consider buying more if they were as inexpensive as they were when I purchased mine.