Benayoun, while not completely replacing Samir Nasri, will help replace him and Cesc Fabregas; he can play wide or in the middle, and has excellent off the ball movement. This is a good signing for Arsene Wenger, but hopefully not the last one.
The Israeli captain is 31 years old, and has played in England since 2005, with West Ham before signing for Liverpool in 2007. He joined Chelsea last season, but only played 6 times in the league, suffering an achilles injury that kept him out for most of the season. In 2009/10, however, he had a very good season for Liverpool, scoring 6 goals in 30 games (11 of them subs), with 4 assists. He also was one of Liverpool's more creative players, with 1.6 key passes per game, second behind Steven Gerrard.
Benayoun makes some excellent runs beyond the striker, as the YouTube video below shows, something that Samir Nasri was doing excellently when he was scoring for fun last season. He's also a better passer than Andrey Arshavin and Gervinho, and should help Arsenal's ability in holding the ball up front.
Yossi Benayoun - Israeli Superstar (via SewellyTV)
All in all, a good signing. Now get Mikel Arteta!