Practice makes perfect. iReal Pro offers an easy-to-use tool to help musicians of all levels master their art. It simulates a real-sounding band that can accompany you as you practice. The app also lets you create and collect chord charts of your favorite songs for reference.
~One of Time Magazine's 50 Best Inventions of 2010~
Used by thousands of music students, teachers and some of the world’s top music schools such as Berklee College of Music and Musicians Institute.
“It’s the perfect technology for a practicing musician: high-quality digital audio, mixable, transposable into any key and completely mobile. Now every aspiring musician has a backup band in their pocket.” – Tim Westergren, Pandora Founder
• It's a Book:
Create, edit, print, share and collect chord charts of your favorite songs for reference while practicing or performing.
• It's a Band:
Practice with a realistic sounding piano (or guitar), bass and drum accompaniments for any downloaded or user-created chord chart.
FEATURES:
Have a virtual band accompany you as you practice
• Choose from the included 50 different accompaniment styles (Swing, Ballad, Gypsy Jazz, Bluegrass, Country, Rock, Funk, Reggae, Bossa Nova, Latin,...) and 12 blues styles available as a one-time In-App purchase
• Personalize each style with a variety of sounds including piano, Fender Rhodes, acoustic and electric guitars, acoustic and electric basses, drums, vibraphone, organ, and more
• Record yourself playing or singing along with the accompaniment
Play, edit, and download any songs you want
• 1000s of songs can be downloaded from the forums in a few simple steps
• Edit existing songs or create your own with the Editor
• The Player will play any song that you edit or create
• Create multiple editable playlists
Improve your skills with the included chord diagrams
• Display guitar, ukulele tabs and piano fingerings for any of your chord charts
• Look up piano, guitar and ukulele fingerings for any chord
• Display scale recommendations for each chord of a song to help with improvisations
Practice in the way, and at the level, that you choose
• Includes 50 exercises for practicing common chord progressions
• Transpose any chart to any key or to number notation
• Loop a selection of measures of a chart for focused practicing
• Advanced practice settings (automatic tempo increase, automatic key transposition)
• Global Eb, Bb, F and G transposition for horn players
Share, print, and export – so your music follows you wherever you need it!
• Share individual charts or whole playlists with other iReal Pro users via email and the forums
• Export charts as PDF and MusicXML
• Export audio as WAV, AAC and MIDI
• Upload your songs and playlists to iCloud for safekeeping and for synchronization across any of your iCloud enabled devices such as iPhone, iPad or Mac
Always backup your songs!

My experience with BIAB was a few years ago. I wanted to learn/experience/practice playing over jazz standards/changes, and got a fairly basic package and downloaded some Real Book type tracks of some standards.
It worked excellently for that. At that time it was mostly MIDI based, and the sounds you got depended on the MIDI sound banks you had available, but with good ones (didn't need to be great ones, I used either a consumer soundcard or a Casio keyboard). I could practice a set of changes at any tempo, load in some pre-entered Real Book changes, or fairly easily type in my own changes. I found it simple to modify the changes to either simplify them or to try chord substitutions. I recall there was even a menu choice to 'jazz it up' a simple set of changes automatically.
As a way to study playing over changes, I can't think of anything else like it, even today. If you looked at the screen you could see the changes, like a scrolling chord chart, and then as your ear developed, you could skip watching the screen. Tempo changes were simple of course, as were key changes, and so I found it superior to the 'Music Minus One' kind of backing tracks of the day.
Eventually I developed what were the height of my 'jazz chops,' which were not of a high level of skill, but which pleased me. They've since deteriorated due lack of practice and some infirmities, but none of that was BIAB's fault.
I never got deep into the capabilities of the program. I was using it to woodshed a hour or so after I had it installed. I did buy a newer version a year so ago and haven't installed it as I'm deep in a long-term, time-consuming music project. A guy I once knew recorded a bluesy singer-songwriter album largely using a newer version of BIAB as 'the band' and those more current RealTracks features. Sounded pretty convincing to me, but I have no idea how difficult that was to do.
If you want a non-software way to integrate the BIAB technology into practice and/or your own loops, the Trio pedal lets you play in a set of changes on guitar (you need to play those changes simply and clearly for accurate results) and it will sense the changes you played and create a bass line to those changes in several styles and play drums (also in various styles) to the tempo (with the tempo being adjustable with a knob). The Trio+ version also lets you record loops of your own playing (fancy as you'd like, like any other looper) to use with the bass and drums, and it also has the ability to set up more sections of a composition. Both the Trio and Trio+ are currently on sale, and both use BIAB technology to do their automatically generated backing.