I recently fitted one of the pumpkin double din and its not bad. Fitting to a 159 was a fiddle - I bought a special fitting kit with a harness adapter, cage and facia as I couldn't initially get the system to work. Its an autoleads kit off Ebay and wasn't cheap at £69. The harness wasn't wired correctly for the pumpkin (or the alfa for that matter) so I had to move switched and permanent live wires around but it wasn't difficult using the old harness and old head unit as a guide but then why fork out for a specialised harness if you still have to mess about? the one thing the harness did give me was a box which converts the CAN signals to something the Pumpkin can understand and now I have steering wheel controls. The facia also doesn't fit the Pumpkin which is a few mm wider so needs to be trimmed. i have made a right mess of that with a dremel so am going to try wrapping it to see if I can improve the appearance. I also found that with the security locking tabs the cage comes with you can't sit the unit far enough back in the cage to sit flush with the dash. I have done away with the security tabs and just bolted the unit into the cage and it sits much better then.
I do still have to sort out the radio as reception is terrible and only locks on to a few stations. A look here (I am posting a question also) suggests that the original HU supplies power to the aerial amplifier or possibly has an amplifier built in which the Pumpkin does not do. I need to find out which so i know whether to buy a special aerial lead which sends power down the co-ax to the aerial or a separate booster.