My god! One that still works!
These terminals had the most unreliable video system I have EVER seen. The brightness is the thing that fails most, leading to having to peer at the screen with all the lights off. Then there is the flyback supression that lead to ugly left-to-right, bottom-to-top diagonal lines. The power switch breaks, and the clips/screws that hold the mainboard to the case appear to come undone. The built in tilt foot breaks, and the tilt and swivel base (if fitted, it was an extra purchase) would fall off whenever yo picked up the terminal.
And this is just the hardware!. The 3151 used IBM specific terminal codes (i.e. not compliant with ANSI X3.64, Wyse 50/60 or any other terminal I came across). Whilst they worked, there were some real ugly features like not being able to turn on or off the bold/underscore/flash capabilities independently. IBM addressed this by having 'compatibility cartridges', which definitely did NOT do what they said on the can. The cartridge for AIX compatibility was supposed to work with AIX (surprise), but in reality, because there were multiple versions, most of which were broken in different ways, it was useless. You had to tweak the termcap/terminfo entries to get them working at all.
And don't get me started in the stupid cables that were the official way to plug them into a PC/RT 6150 or RS/6000. 10 pin MODU or RJ45 to 25 pin D-shell, straight through 25-25 pin serial cable and then a Serial Interposer (that was wired differently from a standard null-modem) that stuck out of the back of the terminal just begging to be broken. (later RS's used 9 pin D shells, a major step forward).
The only good feature was, as jake said, that they came with Model M keyboards (a real class act), but this was spoilt by having a stupid RJ11 connector on the end of the cable that meant that the keyboard could only be used on 3151s without hacking them around. I also agree with vi (pronounced vee eye not vye or 6 [think about it!]), but would say that if it suits you, Emacs still works very well on non-graphical terminals.
Thankfully, the 3152 and 3153 were better, but they were still worse than DEC/Wyse/HP and any number of small company alternatives.
My favorite was a small company set up by some ex. Wyse engineers, called Falco. These were amazing terminals, with good keyboards, readable screens, dual serial ports with separate terminal sessions on each, good ANSI X3.64/VT220 (amongst a host of others), and to cap it all, good Tektronic 4014 emulation. And they were cheap! They were the perfect compliment to System V systems, allowing all of the AT&T goodies such as S, graph, sag and the tek backend to diTroff to work. The only thing that was better, I found, were the Blits (5620/630/730), but these were in a different price league from any other terminal.
Oh well. It's mainly all boring history now, as my work colleagues keep reminding me. Where's the Boring Old Fart icon! I guess a beer will have to do.