I use it as the next step up from a metronome. There are now the "real tracks" that are midi band sounds, guitar, bass, banjo, fiddle, and mando chop. It's good overall practice to not only play lead, also to play good rhythm. There are places on the web to find songs in their format already done. It's great too because you can speed up and slow down, change keys easy. I find it a totally useful program. Once you get your feet wet, you can start making your own songs. I don't know how to gauge how hard it is, it was not that easy for me at first, not coming from a midi/'puter background. But I've got a pretty good idea of it now.
I'm 62 I use it every day. Just got done practicing 4 new Irish tunes I've been working on. It's the best piece of music software I own and not that hard. Go for the full package it's worth it and great tech support if you have problems.
I have not used Band in a Box, but another program you might consider is GuitarPro which I use extensively. I find it very easy and intuitive to use and there is a large library of songs out there already done in GuitarPro format. I've used it to create a full arrangement of Copperhead Road among other things. Most recently I used it to compose a somewhat rocked up arrangment for Morrison's Jig with parts for Mandolin, Flute, Fiddle, Electric Guitar, Bodhran, Electric Bass, and Drumkit - mostly composed on my laptop on a long flight from Heathrow to Ottawa. You can set the playback to any speed which is handy for practicing - my son used it to learn the Bass part and my daughter to learn the drum part, both of which we're recording this week.
I've been using BIAB for about 5 years. It's a good practice helper, like TonyP said; why settle for a metronome when you can have a digital backup band. Makes practicing so much fun it's kind of addictive.
It does present a learning curve. I upgraded to a real-tracks version last year. (Once you try that, you'll never want to go back to midi.) Sometimes you get little glitches, like the snare drum hit at the beginning of this track. (Can't blame BIAB for the bad timing and intonation, that's all me.)
The tech support seemed very good, the couple of times I needed it.
If you have a Mac, iPod or Android, an inexpensive alternative is iReal b: Decent sounding midi tracks, you can change key and tempo. Not as many styles as BIAB; I play mainly jazz and latin, so I'm not sure what all they have.