I got a pair of earplugs from my local Boots (UK) - custom made and they bring the sound down by 15 decibels.
The main reason was a local venue to me that I go to a lot has soudn that is way too loud and will leave your ears ringing.
The guy at boots who sold me the earplugs said that any time you have muffled hearing after a gig then that is the potential for damage. He said he had suffered from nightclubs.
So if you are a regular gig goer then plugs are a must - if you are casual then its a risk but it is upto you whetehr it is worth taking or not.
I have been to the O2 many times and generally don't wear the plugs because the sound there is generally a lot quieter than smaller venues but if Muse are going to play mega loud then I may take mine with me. I guess it also depends on where you stand.
Finally, the earplugs DO make the sound clearer - it removes a lot of the crap/fuzz - and the sound of people taking - it makes the audio clearer.
The main downside is that it remvoes a lot of the bass in my experience and 'the feeling' - which some may find unacceptable.
For me at the smaller cheaper gigs I will wear the plugs and take the hit.
At the more expensive gigs it will depend - obviously if I am paying £71 for Muse I don't want to lose the full experience - but at the end of the day my hearing is more important - so whether I wear them will depends.