Have stayed at hotels on South Beach before but had never stayed in Downtown Miami until recently. I think JW Marriott's virtues are: good location near shops and food, extremely polite staff, outdoor pool, small but adequate workout area, and a reasonable price.
We had a Bay View corner room which was perfectly nice, clean and comfortable. We lounged at the pool and grabbed a workout in the spa, which, by the way is a good place to grab fruit infused water, an apple and a daily paper.
Overall, this hotel was a great value. There are many hotel options on/around Brickell and if you're looking to increase the "wow" factor, try the JW Marriott Marquis, the Epic Hotel, or the Viceroy which are 0.5 mi up the street.