Being in "campus" police work for the last 30 years, I've seen a fair bit of bike theft.
The above post about the types of thieves is right on. Most of our thefts are mid-level mountain bikes locked with cable locks. The tool of choice is the small bolt cutter. You can cut an inexpensive cable quicker than you can unlock the lock with the key.
We have a "program" with Kryptonite and we sell the kids a good-quality Kryptonite lock at 20.00. (heavily subsidized... these go for about 50.00 retail) Over the years, we have lost only a handful of bikes that have been properly secured with these locks.
Why would the thief bother? There are dozens of similar bikes in the same area with cheesy cables?
It's true that heavy-duty tools will defeat even the best locks, but at the expense of noise and attention-grabbing ruckus. An angle grinder is noisy and throws a lot of sparks. If the bike is in an easily-observable public area, this is not going to be a prime tool for the thief.
Nor are bolt cutters with four-foot handles. Most bolt cutters won't even open wide enough to accept a U-lock shackle.
Thefts vary by area. Around here, you can't give away a roadster. Thieves want easy-to-sell or pawn mountain bikes, and not very expensive ones at that.
Somewhere else, a high-end roadster may be easily re-sellable. Here, the pawn shop will look at you funny.
If you have a high-end bike, do not leave it locked up somewhere for extended periods, and certainly not in an area that's easily observable. If you're going to have a daily-commute sort of bike, consider a beater.