I'm late following up on my previous X4150 thoughts - I can blame conferences for that at least - but I shouldn't shirk my responsibilities.
It is very difficult, from a buyers perspective, to decide what that sweet spot is in terms of buying hardware. The current computer market, and more importantly the current computer climate, has completely changed the way that we look at buying computers.
Traditional scale-up models involved buying really big bits of iron and expecting them to do everything. But we outgrew that, so then we tried scale-out, and instead of lots of big powerful computers with lots of diskspace, we moved to smaller, more discrete but powerful machines, with less disk space (which we left to the SANs and the big-irons), but lots of CPU power. All you needed was a machine with a CPU to process work and a boot disk. Small boxes, with lots of computing power and loads of them, hence the proliferation of 1U servers with one or two disks and lots of CPU.
Today, we are seeing a difference. People are looking into consolidation, virtualization and to getting the best out of their machinery in a small, and power efficient device. You need disk space, to store your virtual machines, databases or other information, you need CPU power, to handle the multiple VMs, applications or systems that you are running, and lots of RAM.
Whether the X4150 hits the sweet spot depends on your point of view, but I'm finding it really hard not to justify a machine with this flexibility to anybody who wants a decent machine.
At 1U high you could fit a lot of computing power into a cabinet. With two quad-core Xeons there's plenty of CPU power, and 64GB of RAM gives you plenty of memory to play with. The 8 SAS drives are also enough for you to have a system disk, and a 5 or 6 disk RAID configuration of your choice with one or two hot spares.
Actually, just writing out that configuration shows there is a wealth of potential uses for this box. I've tried a lot of different configurations with my box, including multiple instances of the same application, using it as a full stack, single box solution for different applications (database, application and web server on the same box), and using it as a single box for each of those.
But my favorite configuration is as a consolidation box. This box is easily capable of running with 6-8 virtual machines (of various OS) running at almost full power with a CPU core each. You could dedicate a drive to each VM if you wanted, and as much as 8GB of RAM to each one. Throw in an additional 4-port Ethernet card and you could dedicate them each a network interface too.
That's what X4150 gives you, flexibility. With 8 cores, 64GB RAM, 8 drives, and 4 GbE ports you can choose a variety of configurations to suit your needs, and if you decide to change your mind, you can do so without changing your hardware, just redeploy it.
Flexibility like that in the modern datacenter is a must. Where the latest trends, or the latest application, mean that you need to change your hardware configurations frequently, keeping the same hardware and changing and moving components to suit your needs makes much more sense than constantly swapping around hardware and 'making do' with what you have.