I have not had a problem with unsealed bearings as I generally pull them apart every few months, give them a good clean and repack them.
If you are more of a install and forget type - then I would go for sealed.
One small point and perhaps irrelevant for most people, unsealed bearings (the good ones - Dura Ace etc) run very smooth as they have no seal drag.
@tynan started
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I have not had a problem with unsealed bearings as I generally pull them apart every few months, give them a good clean and repack them.
If you are more of a install and forget type - then I would go for sealed.
One small point and perhaps irrelevant for most people, unsealed bearings (the good ones - Dura Ace etc) run very smooth as they have no seal drag.