I've been scratching an itch to change to a convertible too.....test drove a 335i E93 the other day.
Impressions: roof down - wonderful feeling of freedom and an aural delight
roof up - lots of creaking and groaning from the roof panels (not sure I could live with that)
performance was way below my 330i coupe - definitely felt the extra weight
thinking of 350Z Roadster or XK8......