I've got a pretty in-depth networking background and for my home network rebuild I wanted something less power-hungry than my previous big AMD box running Vyatta, but needed something that supported n+ layer 3 interfaces, BGP and VLANs (i.e a proper router). Couldn't justify the cost of a 1900 series Cisco and associated k9-sec license, so went for a RB1200 (512MB RAM, 1Ghz PPC chip, 10 physical ports, 5 of which can do hardware switching and wire-speed filtering) and a couple of Groove access points.
Getting tagged and untagged VLANs to interact together on a single port is a non-starter (not supported at all), and getting VLAN trunking and routing to work together simultaneously requires terminating the layer three interface on a virtual bridge, then for each VLAN you need to create a virtual VLAN interface for each physical interface, and then put that onto the bridge as a 'bridge port'. The documentation is very scant on this side of things and the command line interface is pretty arcane compared to IOS but does make sense after a while.
If they weren't 1/4 price (or less) than an equivalent product from $enterprise_vendor I'd hesitate to recommend them to anyone, but seeing as they are - if you've got a networking background and can put up with some of it's quirks and limitations, there is very little out there that can compete with Mikrotik on price/feature set.
Positives: Cheap. Powerful. Stable.
Negatives: Harder to use than OpenWRT/DD-WRT or similar. It is still a web-interface, but doesn't "baby" the user. If you aren't comfortable manually setting up interfaces and then setting up tunnels through those interfaces for example then skip this.
I love it. But I won't kid myself, it isn't for everyone. The documentation isn't comprehensive and the software is very powerful but not very intuitive (or at least it isn't if you don't have a good background configuring network equipment).