Mouse preference is very subjective but I personally love the Sensei. I had the Xai for a year, and when it was time to get a mouse for my second desktop to replace my Deathadder (I absolutely hated its high profile and small glides) I picked up a Sensei. Now it is on my main desktop.
A Sensei is basically an upgraded Xai. If you have a Xai and don't need another mouse, no rush to get one. If you need a new mouse I highly recommend it. I love the weight, the shape, and the huge glides.
If price is a huge concern for you then perhaps you should look at Steelseries' Diablo 3 branded mouse. It is $70, has the same rubberized grip as the Xai (which strangely has less traction than the grippy plastic of the Sensei), has the upgraded button switches as the Sensei, and has the same shape and weight as the Xai/Sensei. All are really excellent mice IMHO.