I started out on Bamboo needles and have a nice size collect of wooden needles now. Then one day I needed a needle size but the lys only had it in the metal. So I bought it. I loved it so much I went online to Knit Picks and ordered the set of the metal interchangeable circulars. I love them! The only issue I have is that the needle size starts at #us4 .
My Boye interchangeable set starts at #us2 and goes up to #us 15 where my Knit Picks starts at #us4 and only goes up to #us 11.
Since I got my KP set, I hardly ever use my Boye set. The KP set has a much smoother connection point and the cable is way more flexible .
To anwser one of your questions, I am a slow knitter in general , but it does feel like I can knit a smig faster with the metal needles. I find I have to be extra careful when I lay my knitting down, to make sure I pull the needles through enough so the sts don't fall off.
Now I am working at building up my metal needle collection. I love them!!