CptAsian said it perfectly. I mounted them inside for looks and the front intakes are quite annoying to clean. Have to remove the front grill every time. Also,
I had to install clips on the sides to stop the filter from rubbing on the fan motor cover. The magnetic adhesion is not great enough to prevent this at high airflow.
I somewhat disagree with common sentiment on this now. Linus has a video out about this which matches my personal experiences in that unless you're doing extreme overclocking where every degree matters, most stock thermal pastes will perform almost exactly as well as performance pastes at stock clock normal operating temperatures. Yes, avoid the metal paste. That one has been disproven as better and it effectively ruins your block. I'd use the stock paste and replace it a year after operation simply because the paste could use a refresh and you won't buy stock paste to replace.
Also, if you buy large tubes of thermal paste, be aware that they shouldn't sit around for months on their sides and they do dry out. Consider ejecting the material onto a clean surface and mixing it before application of the paste is several months or a year old and the metal particulates have separated from the non-metal material. You can see this if your paste appears watery around the edges.
I would argue that a properly mixed paste will give you
better performance increases than trying different pastes. Also, the amount you use makes a huge difference. Check Linus' video or other sources for the proper quantity and application method per processor and cooler, etc.