I admit to having been somewhat sceptical of The Time Traveler's Wife at first; "popular" fiction doesn't seem to match my taste. But I tried it and liked it. So, when I saw Her Fearful Symmetry on sale, I picked that up too.
Niffenegger starts off confidently, but I wasn't really drawn into the story, and that didn't really change throughout. It didn't help that two of the key plot twists, formally revealed toward the end of the book, were evident from the very start, leaving relatively little suspense in book meant to be suspenseful.
What saves the book is the characterization. The key characters are largely full, interesting, and carefully drawn. Martin, from the upstairs flat, is perhaps the strongest and best of the lot. Strangely, the twins themselves, while they start well, and are meant to grow stronger, do the reverse; they become less and less interesting and distinct as time goes on.
While the characters are generally strong, there are several key points where they simply act out of character, or shift character for no good reason. It feels as if Niffenegger simply ran out of interest or energy, perhaps deciding to move on and fill in later - but never doing so. Since the characters are the heart of the story, it's a substantial weakness.
The ending regains a little ground, but in this case, there's an expected twist that doesn't happen. That lack, and the lack of an explanation for it, weakens the story again.
All in all, a pleasant, unobjectionable story with some interesting points, but not really outstanding.
* Only thanks to Wikipedia did I note the clever rhyme of symmetry and cemetery, but I like it.