Definitely make reservations if coming on a weekend evening as you might be waiting a long time without one (I've even waited a moderate amount with reservations). There are benches inside for waiting, fortunately. This is probably my biggest gripe with Dosa; if I've made reservations and am paying for a nice meal, don't ask me to wait too. However, once seated, the experience is very good.
They have an uncommon combination here: a full bar with an inspired cocktail selection, an upscale experience at an expensive but sane price, and authentic south Indian food. One of the highlights of Dosa is a very knowledgable waitstaff that have always been able to help me navigate the vegan selections.
Some of the best south Indian food around with a nice ambiance; recommended for dates and adventurous small groups.
Cons: Need reservations/wait time