The number 1 thing keeping me from using it under 7.1 is the lack of an auto-refresh. And then there's the dangerous practice under 7.1 to grab a random port at startup, regardless if it's a dedicated port used by another IBM service. If they fixed those glaring deficiencies (and added MGTC integration) in 7.2, I would consider converting.
I'm getting the certificate warning also and FF doesn't give me the option of adding a permanent exception. Also, mouse scrolling in FF is so slow that it's unusable.
i Access for Windows 7.1 is supported thru Win8 and IBM i 7.2. The big question is whether it will run on Win9.
For speed, of course the Windows client is faster. For most things though, the web UI is good enough for me. Active Jobs takes just a few seconds.