Getting S-Off is not really required if you're just looking at flashing ROM's. I've been using my phone with S-On since more than a year and no problem in flashing any ROM yet. You just need to flash 4ext recovery and enable smart flash. Just follow this awesome guide http://forum.xda-developers.com/show....php?t=1631861
And you may go for S-Off after you get to know more about your phone...
Thanks everyone. Finally, with your helps, I flashed Cyanogenmod 11 on my HTC Sensation, and all the progress went well, without loosing anything important! But, After a few hours of testing, I found some problems, that made me go back to official Rom. I think I'd better write a review about it for other people too.
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