Love this CPU. I have one and it's been in daily use since 2009. My brother bought it back then with an appropriate motherboard and easily unlocked the 4th core and raised the core clock slightly to reach 3GHz. 100% stable for us, never bluescreened, never caused us an issue. In the past few years, it has been in use in my father's system with the same setup going on and it is still mighty fine.
In my eyes, the X3 720 has always been an amazing value processor, truly one of the greats like the Celeron 300A and the Durons. It probably overclocks nicely, although I never had the chance to push it honestly, but the 4th core unlock had a really high success rate. I was so jealous of my brother back then, as I was using an E6550.
I believe at the time (May 2009) it cost around 130-140E, so it was an amazing deal, as Phenom II X4s started at ~220E (?). If you were going Intel, the Q6600 was a nice deal, but probably not as good overall (plus the platform was about to be superseded), although there were also the Q9300,Q9400... and the Q9450,Q9550... The former ones were pretty good performers, although I believe they were closer in price to the X4s, rather than the X3s. The latter ones had twice the amount of L2 cache and were really expensive back then. There was also the i7 platform, which seriously kicked ass, especially with the i7 920.
I don't think the first i5 had launched yet.
It is definitely no slouch, especially if you unlock the 4th core, in which case it should be directly comparable to a Phenom II X4 of the same core clock. If you keep at stock that means Phenom II X4 925. The X3 720 is lacking 512KB L2 Cache for the 4th core, however it still has all of the L3 Cache, so it should be only slightly behind an X4 of the same core clock. I'd say, despite the years that have passed, Phenom II are still plenty fast for everyday use, although if you plan to use one for gaming you will find that you will be severely CPU limited in demanding games from the last 2-3 years.