This whole area is being developed and when completed it should be pretty good, but that may be a few years and until it becomes fully occupied you'll need to drive to restaurants and shopping etc. But you'll have a choice of the puerto or Isla Plana closeby.
Personally I reckon the puerto de Mazarron has a lot to offer generally, the range of different beaches is amazing but the puerto itself gets very full in July and August, mainly with Spanish tourists from Madrid.
I agree with most of Windtakers comments. Buying off plan is a risk anywhere but even more so in Spain, but a lot of the infrastructure is in place in parts of the mar de plata urbanisation. I would still be cautious and highly selective on the purchase.
Sorry LJB15 I disagree with your choice of solicitor. Unless you really know the ropes yourself you need a good solicitor when you buy in Spain and IMHO PALS is not one of them. You say you "had no issues" but invariably these come to light after a year or two. Of the local solicitors in Mazarron, Silvente & Associates are far more thorough and professional.
Bluemed have a mixed reputation, but as with any real estate company in Spain if you ignore all their promises and most of what they say you won't go too far wrong.
I looked at the area and believe that as a generalism it's pretty good and likely to be a good investment - then bought in Isla Plana, but that's just personal choice.