I've been a steady Opera user since 9.x. Prior to that I would check it out periodically but have stuck with it since 9.x (which, incidentally, has my absolute favorite UI of all versions).
However...
Even with the 9.x series, I've noticed that Opera can consume huge amounts of memory if it's available. When it hits just over 500MB, it starts to slow to a crawl. I first started to notice this in 9.x on Windows XP years ago (~2007-2008_. Back then, the machine I was running was a dual processor Dell Precision 690 with 2GB of RAM and a pair of dual core Xeon 5130 processors running Windows XP. I started to notice that Opera's performance would degrade to the point of being unusable when it hit 500MB or more. Closing it and reopening it would help, but it would consistently run slow as soon as it would hit that magic 500MB number.
Eventually I upgraded to 10.10 and, sure enough, the exact same issue would pop up. Over the years I've lived with it, but began to use Firefox more and more. Recently since I've been using it more in favor of Firefox (despite FF's NoScript plugin not being available for Opera) and with my current 12.16 install the memory usage is even more out of control. Maybe 10 minutes before signing up and creating this thread I was at 1.7GB of RAM with only 6 tabs open. One in Outlook.com, one in Gmail, and the other 4 in google searches. I had to kill the process in Process Explorer just to be able to close it. My curren machine is an laptop with an i3-330UM and 8GB of RAM wunning Windows 7-64 bit. No other application I run holds on to this much memory, not even my Solaris 10 x86 VM w/2GB of RAM running under VirtualBox.
I'm at a point of frustration. Will there ever be a fix for this? I've done enough searches and have seen the same issue reported as far back as I've been having this problem. I've disabled as much caching in Opera as possible and yet it's still hanging on to large amounts of my RAM.