I haven't seen that 5.x have been officially released yet, but I have some simple static routes in 4.2.x. They worked fine but it took me a little while to stumble through the CLI interface when setting them up.
I have heard that Nortel is planning on charging a 'license fee' to unlock 'advanced' features like SMLT, VRRP, OSPF, and IP Fix exports in the 5.x code. It still might be cheaper than an entry-level 8600 though.
Of course I'd wait for 5.0.1 or 5.0.2 before putting these new features in production...