I searched and didn't see any good solid info on this, so here's my report.
I subscribed to SmartMusic.com yesterday. It's $36 per YEAR.
Once subscribed, I downloaded the SmartMusic program, ran the install, logged in, and got to work.
What does SmartMusic do? Well, let me describe what I did with it last night.
I searched their library (they claim to have over 30,000 titles) for "Aebersold". They have bunches. I picked Vol 2: Nothing But Blues and opened it.
The program showed me all the tracks from that volume. I picked Slow Blues in F and opened it.
It opened up with a small tool bar and a lead sheet just showing regular jazz chord notation. I selected on the menu to change from the default instrumet of Flute to Electric Bass. The sheet music on the screen changed to showing me a chart with the chords and a walking bass line (on a bass clef).
I clicked on Start and it started playing the music through my laptop speakers. It had piano, bass, and drums.
I clicked on the Accompaniment button/arrow and a drop list appeared and I turned off the Bass. I started it over and was able to play along with just the piano and drums to practice walking bass lines.
When I finished, a small window popped up with choices to Listen, Keep, MP3, or close. I clicked Listen and it played back the song with the accompaniment and the recording it just made of me using the microphone that is built into my laptop. Excellent quality recording! Bad playing.

I clicked on MP3 and it let me choose a file name and folder and cranked out an MP3 of me playing with the accompaniment.
Each time I clicked on Start, it would do it over again and record it as a new Take.
There's a Tempo slider at the top so I could have it play at whatever tempo I wanted. And there's a Transposition button so I was able to change it to play the same thing in G, instead of F. When I changed it to G, the sheet music also changed to display the same notation for Bass except in G instead of F.
It also has a way to set a start and end and have it just loop whatever section you want.
Totally SWEET! I actually ran a line from the headphone out jack on my laptop to an input on my mixer so I could have the accompaniment coming through the same PA monitors that my band uses when we rehearse and I could then play my bass at a "normal" volume and still hear the accompaniment.
They have other charts where Bass is actually the solo instrument, not just part of the Accompaniment. For those, after it records a take, you can get an Assessment where it will actually show the notes you played overlayed on top of the sheet music. Where you played the correct notes, it shows your notes in green. Where you played it wrong, it shows your notes in Red. I have not actually played with this yet, though.
I'm really excited at how easy this tool is for doing something like recording demo tracks of myself. But, even without the recording capability, just the ability to play these tracks, with no bass in the mix, loop it, slow it down, and change the key is totally awesome! MAN, do I wish I had had this when I was in high school! So much better than having to go buy actual Aebersold records on vinyl and play along in the family room!
And the sheer volume of material they have on tap to use to play along with!
If any of you are trying to develop your reading ability, your technique, or your ability to playing walking jazz/blues lines, you should absolutely check it out! $36 a year is a HE11 of a lot cheaper than private lessons or buying play-a-long books. I would have bought 2 Aebersold book/CDs (looking for those is how I found this). I'm already money ahead!
http://www.smartmusic.com