For a quality weekend lunch around St Kilda Beach, and having tried all of the establishments here; I would have to say that Republica is my favourite. Okay sure, the prices may not be the best; however you are receiving great ambience and a terrific view that at times can feel so unbeatable for a beach side location.
Expect friendly service, although I would not say it is overly efficient. A great restaurant to come to some time during the weekend with a friend and cover a lot of ground. If you are after a quick bite; there are plenty of fast and 'faster food' establishments around in Melbourne that will graciously meet your needs with both arms.
Worth it for the great roast lamb pizza in particular. Expect quality, fresh and well cooked chunks of lamb and a beautiful herbal concoction of flavours to boot. And the music is chill out style and of a laid backish nature which is what you would want if you want a lazy hour maximum, and then allow the rest of the day to be productive if you so choose.
And did I mention that this establishment offers beach volleyball tournaments at $20 per person on Tuesdays, Wednesdays and Fridays? Not weekends I know, yet for $20 a pop it is great exercise and you will also receive a free drink and a slice of their yummy pizza.
Next stop: try their Oysters on a Friday, and go for brunch sometime during the weekend. This establishment isn't so great in wet weather, so picking a sunny day to enjoy it is an absolute must.