They've sold plenty of them so they must be OK. IIRC they were the best selling compact MPV at some point or another. My Dad had one and he loved it until some muppet wrote it off for him!
As with any French car just check everything works as it should, including the a/c if fitted, and make sure all the dash functions are working as the dash is a complex piece of kit, (all digital electrickery). Also make sure both remote keys are present and working and if it has alloys check the locking wheel nut tool is present too. None of this should stop you buying one, but a new key will cost you around £170, and if the locking wheel nut tool is missing you can't get a replacement as the code you need is stamped in the key tool itself. This means having the old security bolts forced off and new ones fitted. Barter accordingly if this is the case. On the upside not many Picasso's did have alloys so you should be OK.
If it's a diesel check if it has a DPF, (diesel particulate filter). The 110bhp diesel usually has one and later 1.6 136 bhp diesels too. If it has ask if the DPF fluid has been filled recently and ask for receipts if they say yes. The fluid is around £45 a litre, and it holds 5 litres from empty, it has to go on ramp to fill it, (ideally), and it also needs diagnostic equipment to tell the car's brain when it has been filled. If it's a petrol ignore all that bit but listen for any misfire type noises and rough running. The coil packs are known to fail sometimes. It's an easy fix but again another bartering tool perhaps.
Also check all the interior trim, it can prove a little fragile if little hands and feet have been battering it regularly.
Other than that check the service history, check it starts, goes and stops as it should and doesn't make any strange noises and you should be OK
