Hi Mike,
While it's true that the BR series of recorders from BOSS are not what we consider Pro quality units, whether you should buy one or, another unit like the Br, depends on what you need to get done with it.. For instance, our drummer has the BR-8 and we do quick song ideas on it. It travels well because of it's small size and with all the flying we do, that's a big plus.
So, if your looking for a unit that can be easily taken around with you to work out song ideas,it might be just the thing. However,if you are looking for a way to make records on the cheap, I would look elsewhere.
Danickstr is right that the unit having line inputs allows it to take in line level signals from preamp 's ,or, any other device that produces a line level signal at it's output.
P.S., it is likely that the BR-16 has more than one line input and one of them is for guitar ,while the others can take either -10 unbalanced or +4 balanced signals. Check this out to be sure before you buy though.
Good Luck!!
Ivan...............