I've used the 290 tpi Vittoria open tubulars (white version), they are truly an awesome tire, as far as ride quality, and a real bargain at that price as others have said. The newer version of the Vittoria EVO CX open tubular is 320 tpi, and available in a 700 x 25c as well as 700 x 23. I've ridden the new 700 x 25's a couple of times on my Pinarello, they're freakin' awesome as well.

Challenge is another brand that's making "open tubulars," like the Vittorias they use the same casing as the corresponding tubular, just made into a clincher. Obviously they're not going to ride exactly like a tubular, because they're not tubulars, but they're definitely the best clinchers I've ever ridden. You give up a lot in regard to durability and puncture protection, these tires are all about the ride. Although I've only had flats a little more than average with them.

My favorite of all these "open tubulars" that I've ridden so far is the Challenge 700 x 27c Parigi-Roubaix open tubular(260 tpi). The width of these tires mounted on Mavic CXP-33 rims is actually right at 30mm. I've been using them on my "day touring bike," an '09 Jamis Aurora Elite (Reynolds 631 frame), and the ride is simply amazing. I've also worn out a pair of Challenge Criterium open tubulars (300tpi) on the Pinarello, I'd say they're very similar to the Vittorias.

Here's a pic of the white 290 tpi Vittorias on the Pinarello:



And here are the Challenge Parigi-Roubaix 700 x 27's on the Jamis: