The deal will indeed strengthen T-Mobile, it'll up them to 42 million customers. However, it could do even better; because T-Mobile/MetroPCS appeal most to the quickly growing pre-paid subscriber demographic, they could grow even more - an article I read yesterday said they could pass Sprint as the third largest carrier within two years.
T-Mobile being a GSM carrier and MetroPCS being a CDMA carrier does mean that current customers won't see any kind of service boost from the network merge. However, the companies share the same spectrum bands and both have an LTE plan as their future aim, so it's likely that a unified LTE service is the eventual plan. Of course, a transition like that would take years, so we'll see how well they can manage three separate networks until then.
These teeth are razor sharp...