Magento is definitely the nicest and probably fastest growing PHP-based e-commerce platform out there right now. It's feature rich, has tons of plugins, active development, a huge commercial footprint, yadda yadda.

Unfortunately, it seems like they've moved a lot of features into the commercial version(s), which are rather expensive, especially for larger sites that require clustering (as they're licensed per-machine). We looked at it a while ago and found that given our catalog size (well over 100k products) and expected load, it required too much tweaking to make perform, on top of being pretty expensive. I haven't really taken a serious look at it since and don't particularly like the product or the codebase.

As for certification, I don't put much value in that stuff. If someone could point me to a Magento plugin (especially one that has some decent sales history in their marketplace thing) that they wrote, I'd value that more than having their certification. I've interviewed plenty of people in the PHP world who were Zend Certified Engineers that were less competent than our most junior developer. If you're looking to impress business/marketing folks though, it's probably worth it, especially for $260.

Given that eBay bought them out last year, I'd expect buzzword factor alone to make even basic knowledge of Magento a decent money-maker for web developers/designers who want to milk that tit for all it's worth in the coming years.