in V3 RouterOS's BGP support is very decent. We typically don't have any issues with it! 
Whats nice is a router with 2 gig of RAM (cheap RAM too) can take multiple full table BGP feeds without issues.
Something else that's nice on our Dual Core systems is that while you are receiving the routes, you are only doing so on one core, instead of hitting high CPU while you receive all those, you only go up to 50% (on dual core system, and lower for quad and dual-quad systems). So you don't have the huge CPU issue when you pull those routes.
We had some upstream limit the BGP to something stupid like 128k! Takes 50 min to get all the routes!