Questions like this is are not really suitable for WPSE because they attract a lot of subjective, open ended opinion, for which there is often no right answer.
Let me give you a real world example.
I have used several WordPress E-commerce plugins, both free and premium types, I've also created my own custom solutions in certain cases too.
However I often recommend JigoShop which is free and from my experience I have found it to be a very robust commerce platform for WordPress, that appears well coded and in the instances where I have used it for client projects I've extended or modified its functionality where needed.
If you are also a developer which I'm getting a hint of (?) seeing as this is for a client, then if you know your way around the WordPress API, you too would be able to use something like JigoShop to its full extent and power and in a situation where you need something a little off-beat, dive right in and extend its functionality.
The same could also be said for a host of other commerce type plugins.
Now back to the issue with questions like this, I've completely ignored mentioning Market Press and in place of something entirely different and while that's what you want to know in part, my opinion is subjective based upon my experience and skill level so much so that what I say could sway you one way or another to make a decision that's not right for you, or possibly entirely right.
When you couple that with several people all recommending different commerce platforms, you're going to be in a position of more confusion than certainty which is why questions like this should remain simply Google searches, to see what others are saying about _Market Press_ vs _The Rest_.
You want to look for people who have reviewed Market Press, legitimately, ideally not people who are affiliates of products too as their reviews are mostly biased.
You should also compare the list of features the product has versus other similar products in the market to see if they meet your needs.
There are popular known contenders (and many more) in this space such as,
- Tribulant WordPress Shopping Cart
- and more...
At one point I had wanted to test Tribulant WordPress Shopping Cart, but opted for JigoShop and have never had the need to go back to try Tribulant's solution. I have also used Cart66 and found that to be a good solution too, but again, it comes down to features required, both now and in the future, which is why any opinion you receive here can do you more harm than good.
If you choose a particular plugin over another, then wish to extend the plugin with a unique feature or possible work out how to achieve a particular way of accessing/presenting data then this is where WPSE becomes your place of choice because questions relating to code that have tangible answers, right answers in respect to their functionality are the types of questions we like and the types of questions you'll receive the most helpful answers for.
PS. Just a quick side note, you'll notice when you go to reply as a comment to an answer you are limited by the amount of characters you can write in response. This is intentional and its not really an issue of increased database size either. Its designed for the most part (even if indirectly) NOT to promote back and forth discussion but rather Question->Answer->Clarification (in the form of Comment).