I looked at Monaros too when I was considering replacing the 840 a few months back. While I'm sure it's got a nice engine and good handling, it does look rather cheaply built and a bit bland in terms of styling. Not really what I wanted in an 840 replacement (that was a £65,000 car when new, and the build quality and materials reflect that). And if the standard Monaro is only 333 BHP it's barely more powerful than the 840, which is one of the things I wanted to improve. The 850CSi is 380 BHP standard and tunes to just over 400 easily so that would've been a better option.
Also looked at a few others... 986 Boxster S, too common and not keen on the girly looks; 996 Carrera 4, very nice and very tempting, but at the very top of my price range and fears of the rear main seal going resulting in a huge bill scared me a little; Nissan 350Z coupe, liked this one but again the interior was very plasticy and cheap compared to the 840; even looked at a few TVRs but the reliability reputation is terrible. In the end none of them really set my heart on fire so I decided it wasn't worth the money and hassle and I'll keep the 840 until next summer (unless something catches my eye in the meantime!).