After shopping at all of Poland's DIY finest emporiums extensively over the past few months, I can tell you price wise Leroy Merlin is the cheapest, followed by Castorama and Pratiker and finally Obi, which is a good bit more expensive. Customer service and knowledge of products is pot luck in all four, but again I found Leroy Merlin slightly better.
Leroy Merlin gets my vote, Castorama is badly managed at least in Bielany - Wroclaw it is, they have an annoying habbit of running out of the most commony used items. Its like where's the stock control?
I have spent a lot of time and money in most of them.
Wroclaw Boy, you're absolutely right. You have about 50/50 chance of them actually having what you want in stock. We were told at Castorama in Warsaw that to buy the lacquer they had on display we could either drive to Poznan or wait 9 weeks.