I agree with Gail-- you can cluster very large databases (VLDBs) very successfully, and Windows Failover cluster can be great at providing high availability for these databases.

It's also possible to have a lot of issues with Windows Failover Clustering if you don't follow best practices, take shortcuts, or have issues in your network environment or storage subsystem. That's true no matter how large your database is. There are settings and configuration issues in SQL Server which can make failover slow at times, too-- it does get pretty complex.

So, in short, I would suggest:

* Making sure High availability (same datacenter) and disaster recovery (remote datacenter) requirements are defined appropriately

* Define a build and migration plan in stages with good rollback and a testing plan

* Implement everything incrementally (which it sounds like you're on track to do as it sounds like you're talking about getting HA set up in the local datacenter before moving on)

For the SAN replication itself, much depends on the version of the hardware, the type of replication (sync or async), the communication path between the datacenters, etc. It can be great, or it can cause a lot of problems depending on implementation.