Oh, be careful now, folks. There are a whole bunch of differences between REBUILD and REORGANIZE that you need to be made aware of including the performance differences that Tara spoke of.
For example, for really big tables, you might want to consider temporarily shifting the database to the BULK LOGGED recovery model and then use REBUILD so that you can rebuild the index in a minimally logged fashion. REORGANIZE is ALWAYS fully logged. Also be aware that REBUILD auto-magically rebuilds related stats but won't compact blobs. REORGANIZE doesn't rebuild any stats, compacts blobs (if you rebuild the clustered index or an index with an INCLUDE on a blob), and can always be done online. As Tara mentioned, it does take longer and it also doesn't do as good a job of defragmenting as a good ol' fashion offline REBUILD (can be done online in the Enterprise Edition). Also, as good as it is at defragging, if you do use the BULK LOGGED recovery model, also realize that you won't be able to do a point-in-time restore to the middle of any log file created where a minimally logged operation has taken place. There's also the DROP'n'REBUILD trick to keep the MDF file from blowing out on large table REBUILDs (although I wouldn't recommend that trick for any clustered index or unique index, clustered or not because of potential disabling of FK's and making data totally unavailable until rebuilt... kinda like an OFFLINE evolution).
So REORGANIZE may not actually have less impact especially where log files are concerned. "IT DEPENDS".
There's lot's more to consider before you just make the decision to go one way or another and when.