Search the newsgroup or FAQ sites and there is no end of tools for diagnosing when to consider rebuilding indexes (typically using either normal index statistics or those from INDEX_STATS) so I won't dwell on them here.
Similarly, you can check my import speed tip for some hints on how to speed up the (re-)building of an index. However, from 8i onwards, there is another option you may wish to consider. Its fully documented in the standard Oracle documentation - but its worthy of mention here, because it has not been widely publicised.
ALTER INDEX vmoore COALESCE;
Coalescing an index, de-fragments the leaf blocks for an index as opposed to a full rebuild cycle. In many instances, this may be sufficient remedial action.