Here are my views on Keldeo.
VN noted this very well, Keldeo definitely does very well against offense, while it does slightly worse against defensive teams. Keldeo certainly is a very solid Pokemon on basically any team, and to be honest I haven't faced that many in my time in getting OU Current reqs (i'll post them later), and those that I did face the users sucked very badly, or at least most of them.
Keldeo can basically break through almost all of its counters by itself, with very minimal support. All you need is Stealth Rock and Expert Belt Keldeo can wreck havoc with proper prediction. You can easily compound kill Latias with Icy Wind + Hidden Power Bug after Stealth Rock, which you can do even easier with Latios. Celebi can be hit very hard with Hidden Power Bug (not incredibly hard but it does do a bit) and easily make it weakened very quickly with proper prediction. Jellicent and Amoonguss are probably the very best "counters" to Keldeo, even though Jellicent easily gets smash by the Calm Mind set, which Jellicent basically can't touch. Keldeo is extremely versatile and powerful, and it can abuse its above average speed to basically outrun and take out over half the metagame. Keldeo does have problems breaking through walls however, and it does take it quite some time to work a stall team. Keldeo by FAR does better against offense, especially Expert Belt. I know X5 constantly says how "HO handles Keldeo very well" which was his basic argument for practically every suspect. Hyper Offense does not do very well (or offense in general) against Keldeo. Considering how powerful Keldeo is and how it basically can simply overpower the majority of its counters, especially the light ones on offense, with relative ease. Saying Starmie, Alakazam, even Latias are sturdy counters is a complete overstatement. Starmie and Alakazam are barely checks, Alakazam really has to come in on a double switch because sacrificing its Sash is just incredibly stupid. Starmie, especially offensive ones definitely get torn up quickly, considering most lack Recovery you're not lasting long. I have already explained my situation with most offensive Latias, but for Gengar you basically have to make sure you don't switch in on Icy Wind or Hydro Pump, as otherwise you're basically screwed. But then again Gengar can't OHKO Keldeo unless it is running Thunder. Offense has really only 1 way to keep pressure on Keldeo, and that's continuously keep offensive pressure. But even then, Keldeo does get quite a few switches against offense. Creating a team with all of those checks listed is mad. Most offensive teams only get 2-3 "light" checks against Keldeo, and by that I mean they get broken up very easily.
Keldeo can kill its own "counters."