Most of the complaints I've heard in regards to XenServer vs. VMWare is the fact that XenServer does not allow an overcommitment of memory. If you have a host server with 16 GB of RAM, you can only allocate up to 16 GB of RAM for the guests (yes, I know, technically less since Xen requires some resources... but you get the point).
Personally, I'm not a fan of over-allocating memory. I actually like that with XenServer, I do not have to worry about it. I've seen too many VMWare farms suffer because of assigning too much memory then having to constantly swap resources. I think if you engineer the solution correctly, and size your servers properly, memory allocation should be kept in check.
However, seeing this as a fault when comparing virtualization host platforms, Citrix has released the latest Beta of XenServer 5.6. One of the new features IS allowing your to over-allocate memory... they call it Dynamic Memory Control. I have to admit, it is pretty cool and very easy to use. The XenCenter display makes it VERY intuitive... all the better to see WHERE your resources are allocated, especially when squeezing memory.
To read more technical details, see my full article at: http://www.thegenerationv.com/2010/04/xenserver-56-preview-part-1-dynamic.html
