I`m moving a UM installation over to a RB 1100.
It will also be a main router.

How many users do you recommend with this setup before moving on to something more powerful?

How many users/sessions do you expect to have? I'd recommend running the UM on a standalone RB or x86, not something with routing responsibilities.

We were running the UM v4.5 on an RB1000 core router. As we converted users over to PPPoE the UM was tracking up near 500 sessions. Running reports in the UM would hammer the processor on that core router.

We built a light x86 server just for the user manager. It's a dual core atom processor with a SATA dual CF raid enclosure. Still not massively powerful, but when the UM hammers the processor it only hammers one core leaving the second mostly idle. And that server is not responsible for any routing, so not such a big deal.

The UM (now v4.11) handles the radius requests just fine, but the web interface is still a bit sluggish and running reports is pretty hit and miss (just sits on "Loading...".

I'm hoping v5.0 speeds things up all around, but I'm not gutsy enough to upgrade the server in-place to RC1.