I have the Samsung Epic Galaxy 4G and it works just fine on my phone. You just gotta remember that just like with any phone you're using a program and if you're in a bad area with spotty reception it's going to be laggy just like anything else would be.
Chat works fantastic, Calling does what its supposed to do albeit a little laggy sometimes with the newer updates. Much better than my experiences with Tango or Oovoo!