This topic comes up a lot and depending on who you ask there is often much misinformation when you compare the two products particularly when it comes to cost and features. Microsoft claims ESX costs way too much but when you do an even comparison they are fairly balanced. As far as features go ESX is way ahead to the pack and is overall a much more mature product then HyperV. HyperV is OK for a 1.0 product and will improve over time as Microsoft releases new versions with more features.
I’m a long time ESX user but keep an open mind toward other virtualization products. For me ESX is the best product available right now because of many features, maturity and the large amount of automation and management products that are available to complement it. If you are not already using a virtualization product and are looking to my advice is to try them all out and make your own comparisons, once you have price them for your environment and choose the one that meets your needs and works best for you.