What sumwun said, basically. You're acquiring freedom of move choice, or lack of life sap, in exchange for only a mediocre boost by comparison to choice band/specs/life orb.
In very, very selective circumstances, it may come in handy, but it's generally not recommended to run it - most don't like the risk of running it. There are countless examples you could use, but if you're planning on using something with a belt, make sure the Pokemon that has it has:
a) a moveset designed to cover a LOT of types super-effectively, and is either:
b) somewhat capable of surviving hits from a good portion of the meta, or
c) fast enough that that shouldn't be a problem. If it's me you're asking, I prefer this one.
After that, I would recommend checking going to Smogon's damage calculator with your chosen set. See if said Pokemon you choose, with this set, provides answers reliable enough to your liking to the threats and opponents you're expecting, compared to the three offensive items listed above. If you decide that the risk is runnable after these, then sure, it can be good in competitive, but let me tell you, it's pretty rare. Especially in OU.
TL;DR: Ehhhhh, rarely. It's enticing, but only occasionally useful.
