As a performer, the Raptor X doesn't differ from its window-less siblings except for on one point: Meant Time Between Failure (MTBF). The 150GB Raptor drive has an MTBF rating of 1.2m hours, whereas the Raptor X has an MTBF half of that (600,000 hours). However, this shouldn't be too great a concern as both drives are covered by a five-year warranty, so it's only once this has run out that you might want to be concerned if you're still using the drive some 60 years on. The only reason I could see for the lower MTBF time is that the plastic window doesn't dissipate heat as well as a metal lid would do. WD has compensated by making one of the sides of the hard drive into a heatsink, much like a high-end SCSI drive has.
You might be disappointed to find out that the new breed of Raptor drives are only 1.5Gbps SATA 150, not 3Gbps. This is because WD is mainly targeting the drives at the SME server market and the SATA II platform seems to have been deemed not mature enough for this segment. This is unlikely to be an issue anyhow as there are no performance limitations here for a single drive as there is plenty of bandwidth available. At least the 150GB Raptor drives are native SATA rather than utilising a bridge solution as the previous models did.
It's quite impressive that the Raptor X didn't make any more noise than a standard SATA drive, especially if you consider how much noise most 10,000rpm SCSI drives generate.