Hi Steve,
The Mk1 was released in June 2010 and was only on sale for about 4 months. It required the pole to be modified to fit so, responding to customer demand we developed the Mk2.
The Mk2 was very successful throughout the rest of 2010 and 2011 (there are many still in use), nothing was 'amiss' but we looked for a way to improve the method of fitting to the pole (a separate part and clamp were needed with the Mk2). We did have an unforeseeable issue with one batch, which we replaced for any customers affected free of charge.
The Mk3 was released early in 2012, being lighter and much easier to fit.
The latest version of the Mk3 has changes to the components just to make it lighter, as that was the main requested change.
They all work in a similar way and we only really see problems when they aren't kept clean, just like a pole works better and lasts longer when it's looked after. We put them through a lot of tests, both in the workshop and out in the field before they're released (see the video on our FAQs > Technical Info page for an example).
Unfortunately as I don't know who your friends are or how long they did use their Mk1s or Mk2s for I obviously can't comment on why they've had any issues. I can only say that we are very confident in the product and encourage customers to contact us with any issue they may have. We have a 28 day money back guarantee for anyone who's unsure to try one and send it back if they don't get on with it.
We've got many happy customers (some of whom were originally very negative about the product before they'd tried it!) and there are now 1000s of Aqua-dapters in use around the world

Paul