Check some of the past posts -- there's been some discussion about Cards as well as GI, and other IM fellowships to a lesser extent. Cards is still extremely competitive, but if GI hasn't already surpassed it as the most competitive IM fellowship, it soon will. Two reasons: 1. Sheer numbers -- there are something like 650-700 Cards spots and 250-300 GI spots, and 2. Lifestyle -- both have nifty toys and procedures but you won't have to be up in the middle of the night nearly as much with GI as you would with Cards (unless you exclusively do something like EP or Echo -- but again, that would mainly depend on the practice you'd join and how the work was divvied up).
I think another factor is the number of years -- general Cards is 3-4 years after IM (research year) and then EP is another 1-2 (some places let you fold in the first EP year into the final year of general), Intervention is rapidly becoming 2 years at many places, ECHO is one-year, heart-failure/transplant is 1 year at least. All said and done, worst-case you could spend up to 5 years after IM residency doing fellowship. This is kind of ridiculous. Given the fact that other specialties are beginning to cut the number of years needed for training (look at CVTS, Plastics or Vascular where the number of required Gen Surg years is being/has been curtailed), I think (hope) you will soon see the same thing happen in these IM fellowhips that go past two years. If for any other reason, funding pressures will probably be the first impetus.