I work near Merkato and end up going there for lunch at least a few times every month. This place is exactly what I expect and want from Ethiopian dining: quiet, delicious, large portions, and cheap. The wait staff is always very friendly, helpful, and accommodating, and the food quality has never disappointed.
I've never had any of the meat selections, but the variety in the vegetarian combination platter is more than enough for two people and it is always diverse! In addition, some of the items in the vegetarian combination platter change around on a regular basis which makes it more interesting if you plan to come back (and, if you are dining with others, it makes for fun conversations/debates about which items are best).
It is, however, a very slow-paced (but relaxing!) atmosphere, so I wouldn't recommend going to Merkato if you have less than an hour available.