I am really happy with Instant Gaming, but want to be totally honest about the pros and the cons!
I will start with the negatives, which is that VAT is added on top of the listed price, so it isn't quite as much of a saving as it first appears. I assume as the website operates in many counties all with separate taxation rules. Also, there is a fee to pay via PayPal, which is always my preferred method when shopping online for safety reasons. I also think that it needs to be made clearer about DLC downloads, this didn't affect me, but I have seen negative reviews about people less versed in the gaming world getting caught out.
That being said, my personal experience was really positive. I bought two DLC for less than one full price on Steam. The codes were instantly in my account and were super easy to add to Steam. They worked perfectly and I have some cool new stuff in my game. I will 100% use Instant Gaming again and am really happy with my purchases.