In the Thin Slicing post:

[...] Unfortunately, that’s where the similarities between [Amagami and Seiren] ended. One, Seiren has some god awful character designs and animation. The heroines all share similar faces and body types, so it is very difficult to tell them apart. The poor animation and lack of detail contribute to to this issue. [...]

Two, none of the characters of Seiren are as interesting as the Amagami cast. They are all bland, background characters forced to play leading roles. Three, the plots are excruciatingly bad. One focus of the first heroine is that the guy thinks she is dating an older man just because she was alone with an older man at her place of work. That is some mental gymnastics going on there. [...]

If what he says here is grounded in fact, it is quite damning.

UPDATE: Evirus is not happy either (even awards it a "Worse Than Cosprayers" tag), but neglects to exaplain why, except the stock complaint about every lead except Cobra:

Seiren manages to be a fairly serious disappointment even though I never had high expectations for it. Its Amagami SS predecessor also had numerous flaws, but I don't remember them being so irritating. [...] Seiren, on the other hand, manages to give its male lead all the traditionally detestable characteristics stereotypically associated with Potato-kun. It's actually too aggravating to go into detail again, given I've already vented about him via IRC and the Twitter.

Twitter killed animeblogging.