I have used imo and almost all other video calling apps. The experience I had with imo is not positive though. The video call quality is not good.
Previously I thought it is because of my poor data network but when I had wifi connection, the problem remained the same. The call stucks very frequently and sometimes I could only listen to the audio. However it is good for normal texting. Voice calling is not too great but even not bad though. But as far as video calling is concerned the performance is average.