I could go on for weeks about the pros and cons of these groups but in short it comes down to your situation and what you need, your capability to pay for it vs do it yourself and about 50 other criteria. My company offers such services to a select few and we have actually built mobile applications for some startups on this formula.
In general, if you can pay $$ to get your development and other work done, do it yourself. It complicates IP ownership, patent-ability, code base maintenance and many other factors. If you do decide to do it, I would be happy to offer some consulting at a reduced rate on what to consider and how to mitigate risks. If you do this the wrong way, it can greatly impact your exit strategy options and even your MC.