We use Twins extensively in our data center and have several racks full of them. We've been using them for several generations and are pretty pleased with how they have evolved over time. We now use the Twin2 units pretty much exclusively. We like the shared, hot-swappable power supplies and 4 systems in 2U layout -- which is certainly dense enough for our needs. We also have a great local VAR (greater Boston area) who is awesome in terms of RMAs, warranty service, and no-nonsense quoting when we need new systems -- they set us up with a login on their web page that will get the price dead on so we can get approval for that amount ... no "we are running a special 50% discount just for you" that requires a phone call and/or meeting. They will also send out guys to do a rack and stack who are really good at it -- you get systems shipped, they will put them in the rack in serial number order (easier RMA when you can just count up!) The prices are also very reasonable, and the extra bit of space for a card allows us to add an expansion card as we need it. We'd been through some other server vendors and we have stuck with these guys the longest because they work hard and are great to do business with.
We looked long and hard at blades, too -- but in many cases they were simply TOO dense for our needs, as we do sometimes need an expansion board, USB slot or some other thing on one of the machines, where we don't have to go up to the full 1U or 2U server to accommodate that need.
We also get into cases where we need traditional 1U/2U systems for something or other, and we generally just use the same guts that are in the Twins, which means we don't have to deal with weird driver issues for a different system board, so we can deploy or base operating systems + packages onto it without issue.
My day job is in HPC land, so I know there are more dense things out there, but for x86_64 computing, the Twins are really good for what they are. When you start stepping off the "mundane mainstream server" path and get into the "ultra specialized, boutique" stuff, the cost starts rapidly outstripping the benefit. Of course, for some applications you need to go there, but for what we do, we have more flexibility with space than we do with budget so the Twins strike a nice balance.