Accuracy of Omni drive?

[teslalab2]

Hi all! I'm building a robot with my money to finalize my arduino-wifi control system. Before I spend my own money on a lemon I want to know what drive system I want going to go with. I was wondering if any one had experience with 4 wheel X shaped omni drive. like this http://i.ytimg.com/vi/Z0lUTMBbu9A/maxresdefault.jpg

I don't want to spend money on encoders. I programmed encoders on my teams bot this year for mecanum, I just don't want that headache for this

So basically, can I even drive in straight line with this, or is it just as bad as encoder-less mecanum?!

Thanks