It is an ok set but the downside is if one of your defenses are not boosted while you are boosting the other you can take severe damage.
A good Registeel set I use is:
Registeel @ Leftovers
Ability: Clear Body
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
- Curse
- Iron Head
- Sleep Talk
- Rest
With this my Sp. Def is very high, allowing me to set up my curse to boost Atk and Def. Rest for sustain and Sleep Talk to gain more stats or attack during sleep