Yea, iirc, Rowley works in OSX.
I tried Rowley, and liked some of it, a lot. But it wasn't going to work for me. I pretty much had to spend lots of money somewhere either at Segger, Keil, IAR, other, and chose Keil. uV5 isn't terribly high end in terms of IDE, but it was well and hasn't crashed or done anything dumb yet. I crashed Rowley twice in two days, but I don't really hold that against them.
Someone mentioned to me Rowley was great for Hobbiest that wants a full IDE and for some reason doesn't want mbed. And someone like a professional hardware guy who wants to confirm designs.
If I was the later or just worked mail with hardware I'd go with Rowley over Keil, but since I do more software, RTOS, production firmware etc, Keil was the better choice for me. I know one thing for sure, setting up a custom toolchain was NOT the right move and I'm glad we skipped that potential mess.