I have an onguard, this one I believe http://www.onguardlock.com/?page_id=329 and I'm not too happy with it. The lock seems to work alright, occasionally it gets jammed, but with a little jimmying and patience it can easily be fixed. My real qualm is with the lock mount. The mounting bracket lost a chip of plastic after 2 months, and now my lock is quite loose in the bracket. The metal component of the mounting system on the u lock has to be tightened with a multi tool every once in awhile (I learned that when mine fell off). I'm considering writing a letter of complaint to the company when my mounting bracket breaks. And fwiw, the mount is on my seat post because of my bike geometry. I've only had 2 heel ejects with the set up so far.

All in all if I were in your shoes, I'd spring for one of the better kryptonite locks. I can't speak for the quality of the walmart ones though.