I have two OnGuard locks. The largest one (which weighs MORE than five pounds), and the "mini" size that is about two pounds. Most of the time, I use only the "mini", locking the rear wheel at 9 o'clock to a sturdy rack. I use an allen bolt and cable on my front wheel.
Based on tests in Europe and the UK, OnGuard makes top quality U-locks. Both legs of the U are locked, just like Kryptonite does with its NY lock. (Only ONE foot is locked on most Kryptonite models).
The only disadvantage is that the key will not turn until both feet are fully engaged. On my "mini" lock, I need to give the cross-bar a "slap" to get both feet engaged. They come with five thin, flat keys. Do NOT force the key, as they will break if forced.
Of course, the lock is NOT the most important factor in bike security. Selecting a high visibility, high traffic location, sturdy rack or locking post, and combining two or three types of locks greatly improves your security. I combine the OnGuard with a cable lock because crooks are lazy. They are unlikely to bother with a bike with two locks when so many bikes have just a $5 "toy" lock, or a good lock attached only to the front wheel, which has a quick release axle.