First, Exchange clustering requires Windows Server 2003 Enterprise Edition or Datacenter Edition for each system in the cluster, as well as the more expensive Exchange Server 2003 Enterprise Edition (only one license of Exchange is required). The additional cost for the required versions of Windows Server 2003 and Exchange will depend on your relationship with Microsoft, but retail is about US$2,000 more for each copy of Windows Server 2003 Enterprise Edition and about US$2,200 more for Exchange Enterprise Edition.
Second, hardware choices are limited if you want Microsoft support -- see windowsservercatalog.com for details on supported systems. The difference in price between a white box server capable of running Windows Server 2003 and a certified cluster member could be an order of magnitude. All systems in the cluster must be identical, and they are certified as an entire system, which means that replacing failed hardware must be done with the same parts that were initially installed, assuming you can find them. You must also have shared disks that all servers in the cluster can access, which means either shared SCSI -- for two nodes only -- or a Fibre Channel array.
Third, Windows Clustering can be difficult to set up, although tools have improved tremendously since the days of clustering on Windows NT 3.51. The two fail-over products tested in this review, SteelEye LifeKeeper and Neverfail for Exchange, are substantially easier to set up than Windows Clustering, although unlike Windows Clustering, they don't support more than two nodes. (SteelEye does support more than two nodes for other applications, but not for Exchange, due to Microsoft recommendations.)
Finally, Windows Clustering does not protect against corruption of the Exchange Data Store, so you'll still need a backup and archiving solution to protect against this type of problem.
If you have multiple Exchange servers in your datacenter, Windows Clustering would allow any one to fail or be brought offline for maintenance without interrupting service. But at lower price points and less complexity, the products reviewed here may make you happier.