The 6150 was only marginally underpowered when it was first released, but suffered from lack of upgrade until close to the end of it's life (about 5 years) when the 6150-135 deskside and 6151-115 desktop systems were produced. The product numbers followed the IBM PC line (the original IBM PC was a 5150), and the initial development was for the PC division to produce a RISC based PC.
But without the 801 processor (which has been argued by some as being the first commercially available RISC processor), there would have been no RS/6000, PowerPC, RS64, POWER, or CELL products (or, in fact, the 9371 deskside mainframe, or some of the current crop of zSeries systems that use POWER processor offshoots).
I always thought the the dial and button boxes were quite a neat idea, and I saw them used to great effect with CAD for zoom and pan operations. How well they were used was largely dependent on how much effort was put into installing it into the workstation (desk - not computer).
What you must remember was that this was an 80's designed system that looked its era, and should have been updated more frequently than it was. I can't remember how Sun 2 and 3 boxes looked at the time, but I'm sure that it was much less slick that the Sparc pizza boxes that appeared in the later.
Whilst it was possible to have async terminal and graphics head attached, it was not necessary. I had a 6150-135 fully populated with 24MB of memory (although it was only supposed to support 16) and 930MB of ESDI disk, with a Megapel adapter and 5081 model 1 17" display as my home UNIX system for several years.
The version of AIX sucked more than a bit (it was a non-paging SVR2 port in the days of SVR3 and BSD4.3), and it was built on a hardware abstraction layer called the VRM which isolated AIX from the hardware (for disk and memory allocation [the VRM did the paging providing AIX with a larger address space than the available memory - possibly the first Hypervisor outside of a Mainframe environment] and serial port configuration, anyone else remember the minidisk and devices commands).
Boy, was it noisy, and the 5081 screen was sooooooo heavy (it had a lump of concrete in it to counterbalence the weight of the display tube). I gave it away to a computing museum (complete with a full set of 30ish install floppies and manuals) when I decided that Linux was a better way of having a UNIX-like OS in the house.