I 've had a full version of Amplitube 2 for some years now, and yesterday I took the plunge and installed the free version of Amplitube 3.7.
The idea behind the Amplitube Custom Shop is that you can try and buy specific amps, cabs, mics and effects in exchange for credits which you buy for real money. Cost of the credits reduces as you buy in bulk and if you have any IK Jam Points accumulated you can use these towards up-to 50% of the cost. I was interested in a few bits and pieces so I hit the CS button within Amplitube and was presented with the online shop. You are allowed to trial anything you fancy for 2 days after which you can't try it again for 2 months, which seems fair enough. I did grizzle a bit at having to select items individually and then confirm that I did indeed want to try each of the items - not too much of a struggle for a couple of amps but when you want to try a bunch of 4x12s that are spread over 8 pages it gets tedious (there may be a slicker way to do this but it wasn't obvious to me - I'll happily stand corrected). All of the items you select for trial appear in your Amplitube equipment list with a countdown timer to tell you how long you have left. I found the mics and cabs I selected straight away but the amps appeared to have gone missing, - they actually appear under their original bundle headings in Amplitube which makes side-by-side auditioning a bit of a faff. Rather than face the frustration of the entire preset list every time, I have saved a standard set-up of each rig (basic amp, preferred cab and mics) to a separate folder with the proper amp name against it and this appears in the preset dropdown list. As a by-note you have to keep the web page open as you use the trial models, not an issue for me but it means that you can't evaluate without a live internet connection.
Prices in the Custom Shop range from 5 credits for mics and cabs to 20 for amps and VAT is added onto the prices you see. Note that many of the models you can buy are included in other bundles, so if you are thinking of buying the full version of Amplitube or the Metal or Hendrix or Fender packs do your sums before you start buying individual models. I just wanted a couple of mics and cabs and a specific amp so I bought some credits, swapped some Jam Points (you do this on the payment page, it isn't particularly obvious before then but it works) - again note that VAT is added to the cost of the credits, and off you go. It's a bit like spending monopoly money so be-careful. Slightly frustratingly, I have managed to have 4 credits left over which is just enough to not buy me anything; perhaps there'll be an offer on some time soon.
Oh - and the new Engl Powerball model is amazing!