@Impel:
Szayel wasn't removed and then re-added. He was just not, at that time, strong enough to get in, and when Neliel left that would have raised him in the pecking order anyway.
I didn't read the original Japanese, but both the MangaShare and Ju-Ni scanlations have Nnoitra specifically say that Syazel isn't an Espada anymore during the flashback. Meaning he was at one time.
Here's the MangaShare scanlation page:
http://www.onemanga.com/Bleach/295/01/
Here's the Ju-Ni scanlation at ReadBleach, I can't directly link due to the way the website's designed, but it's chapter 295, right after the intro pages on page 4.
So yes, unless both those scanlations are wrong, Syazel was an Espada and then lost his position before becoming an Espada again.
@Geg:
He could have been an interesting character. We knew virtually nothing about him before his fight with Byakuya.
I liked him the moment he went to kill Rukia, if for no other reason than the fact that all the other Espada kept on going on about how Rukia "must be dead" without bothering to make certain of the fact, and he actually said as much. It was nice to see one of the Espada show a bit more caution, and not jump to conclusions.