After using Analog Factory for a while I found that the quality of emulations is superb. You have a reasonable amount of control with the usual ADSR, LFO, VCF and other controls available. For each patch there are also three controllers which have been assigned to whatever the most useful additional controls would be. These are usually well chosen and allow considerable changes away from the original patch. All control changes are smooth without audible glitches or zipping and the Arturia technology that emulates down to the transistor level on a particular circuit board is stunningly accurate at recreating the original analogue feel. I found that the controls were already mapped to sensible settings allowing immediate use with a controller keyboard (Axiom 61 key) and Logic. The only downside is the sounds can sound a little too clean, which can easily be remedied in the mix. There are two standard effects built in; delay and chorus which work well with the traditional sounds on offer.
Patches are selected using a straightforward and fast category grouping table, which allows you to quickly narrow down your selection. There is a useful "favorites" option which enables you to group and quickly select your preferred patches. There are also shortcut buttons to user selected patches on the main interface.
All categories of sounds are strong and useful with some stand out sequences (arpeggiator) patches which automatically match the tempo of the track when used as a plugin instrument. These are accurate emulations, so the polyphony is often limited to what the original instrument would have managed.
Analog Factory is a great way of having a huge arsenal of accurate analogue sounds at your disposal and represents stupidly good value.
Pros: Price, quality of sounds, range of patches, easily tweaked patches
Cons: fairly heavy CPU usage for multiple instances of Analog Factory, can take a while to load each instance.
Test computer: iMac 2GHz Core Duo, 2gb RAM, Logic
UPDATE: there appears to be a bug with Leopard 10.5.2, Logic Studio & Express 8.01 - saved projects with Analog Factory 2.0 instruments stored inside will cause Logic to crash on loading. Somewhat annoying. There's a work around until this is fixed: hold control down when you launch Logic and disable the Core Audio driver. Then load the project and switch on the Core audio driver by clicking enable in Preferences > Audio. Tedious, but at least it works.
Update 2: This appears to be fixed in Analog Factory v2.2.1 running on Logic Pro 8.