Got a 2007 vito at the moment, company van. Seems ok, not sure of its previous history so can't say. Not overly refined, fairly good on fuel, nice size for driving, but lacks space for install type work. All Mercs rust and my gut feeling is the build quality isn't what you'd expect from a merc. Had a 57 plate vw crafter for 3 years previously, it's just a sprinter with a different lump. Had endless problems with it luckily under warranty but wouldn't buy one myself. Put me off merc quality. Had 2 vivaros which are lovely to drive, don't rev very high and have their own common faults which can be dear to fix. Gearboxes on the older models and injectors seizing into the head on the newer 2.0 models.
used a 2011 transporter for a bit, nice van although it was brand new, bit on the small side compared to transit or vivaro, depends what you use it for.
got a transit now, 2007. Nice to drive, quite refined.comfy seats, not the quietest in cab, rust easily, cheap to fix and semi high top means tube carried internally.
Also had a 2006 relay in the past, nothing much mechanically went wrong with it but the body work was beer can quality, doors kept needing adjusting and other annoying problems. Think that's it for recent vans.
so, from my experience they all have pros and cons. I'm selling my transit soon as I have the company vito and getting a 174 transporter as they look better and will be use more for recreational use than work.
