I can't find any threads on this but I'm sure there's a lot of people using aftermarket batteries as opposed to shelling out 80,- for a 'simple battery' by Canon...
Any tips on this one? Particularly interested in experiences of people using the third party batteries 6 months or longer. (How) do they hold their charge? Do they communicate with the camera and charger? Are they, in short, a viable alternative to the Canon batteries?

They don't last, that is why they are cheap.

I disagree. B&H, for example, sells aftermarket batteries which are not cheap and -given their reputation- I can hardly imagine they're selling bad batteries. Since I live in Europe however, I'm hoping for good eBay deals... (can't find the B&H brand on eBay unfortunately but hoping that someone knows them by a different name perhaps...)

Just because B&H sells third party batteries doesn't mean that it will last, you ask for people with third party batteries experience, I gave you mine and it is not what you hear so you now are disagree with me. LOL

I disagree with your generalizing, that's all. I have bought aftermarket batteries for my videocamera's in the past and they were great, particularly those from B&H.

I have used Pearstone batteries for my 5DM3 and my 60D (same battery) for a few years and never had any issues. I did have one that didn't communicate with the camera correctly and B&H swapped it out no questions asked. Also use a dual charger I got from Amazon with no issues too.
Amazon.com: DUAL-ion+ ReVIVE Series LP-E6 Camera Battery Charger for Canon EOS 60D / 7D / 5D Mark II / 5D Mark III Digital SLR Cameras: Camera & Photo (http://www.amazon.com/gp/product/B0049CPQ36/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1)