I wound up watching SYD because Author did and liked it. I liked it a fair bit; it was not deep, but the jokes reliably made me smile and even laugh, and they were (mostly) a style of humour that I enjoy.

As characters themselves note in episode 3, the humour generally really does need the straight man (usually Takatoshi, although Hagimura sometimes fills this role). I'm not going to try to figure out just why, but without the straight man many of the episode 3 jokes just feel flat.

One thing I liked is that SYD is willing to be subtle. There are several jokes (and in fact a long-running series of little ones) that you have to be paying attention in order to understand. If you didn't pay attention to one of Aria's absurd offhand comments in an early episode, for example, one of the amusing bits in the third episode is just opaque.

The 4-koma base of the show was very distinct here. However, it somehow worked for SYD when before it totally didn't work for me in Lucky Star (I bounced off LS because I could practically see the strip boundaries). My best idea of why is that it's because usually any particular joke was over quickly and SYD moved on rapidly; extended sequences were rare, so if one joke didn't really hook me things would soon change.

(It's also quite possible that I liked the style of humour better than what I remember as Lucky Star's less punchline-oriented jokes.)

Finally, it should be noted that SYD is not all sex related jokes (although these do dominate); the girls hold forth rather frankly on a fair range of things that generally aren't discussed in 'polite' society or mixed company.

Liked: definitely. I laughed a lot.
Rewatch: maybe. I'm not sure how funny it would be on a repeat viewing. As with Author, I can imagine (re)watching this at an anime club (although it would take the right audience; with the wrong audience, this is the sort of subject material that could go horribly wrong).

(Sadly, I believe that all of the Toronto anime clubs are defunct.)

Update:

Author:

Still, I can hardly credit the notion that borders of 4koma strips were any less noticeable in Seitokai Yakuindomo [than in Lucky Star].

Oops, I was unclear. The 4-koma boundaries were, if anything, more clearly visible in SYD than in Lucky Star (Author outlines a typical example in his entry); however, SYD worked for me anyways. I don't entirely understand why, but it surprised me; when I started watching SYD and realized not just what it was based on but how little effort it made to disguise it, I expected to bounce off of SYD even more strongly than I had bounced off the milder 4-koma based Lucky Star.

(That I bounced off Lucky Star is something that I regret. It's clearly a good show, it's just that I seem to not be able to watch it. Maybe someday I'll have a chance to try again.)