I for one have had 1/2 price oysters at Shaws more informal Oyster Bar countless times and never had a bad or off oyster.
7 days a week, between 4-6PM all oysters are 1/2 off. Considering how expensive oysters are, this actually puts them in the reach of an average guy's budget, and if you get 3-4 people together allows for trying a lot of different kinds of oysters, which may otherwise be price prohibtive for some (me included).
I have always had a good experience during the "Oyster Hour"(s), and the oysters are clearly coming from the same oyster pile
(by the bar) as what people get when they pay full price.
Even better, up until 2 years ago, one was limited to a single variety of the day for the deal--now, any variety they have listed is fair game, and I often go for the usually slightly pricier (briny, aggressive, clean finish)Wellfleets. When I want a big meaty oyster, I get the Penn Cove Selects.
Its not just Chicago, but I've noticed that in Boston, as well as at the Oyster Bar in NYC Grand Central Station (a must go if you've never been), and the Oceanaire in D.C. as well as Dallas, Wellfleets are a little more expensive then other varieties. Not sure why, but they are.
Also if you are lucky they will have a good live music band at the Oyster Bar at Shaw's.
2 tips though:
1.if you don't ask for, or mention the deal, your server may not mention it unprompted. I hope that its not because they fear the tip getting slashed, but there is a pattern of remaining "mum" on the subject.
Once mentioned though, there is no resistance from the waitstaff.
2.Check the bill-- last time I went server mistakenly charged full price, but quickly and apologetically changed it when it was pointed out.
Get out there and enjoy some 'ersters to your heart's content! Now that the Oyster Guy from Fulton's has gone to the suburbs, beyond any doubt Shaw's is still the go to place downtown for good oysters on the half-shell.