Hiya
I change my nails every 2 weeks, whether I have iLac on or Shellac. I find the iLac lasts just as well, provided I UV cure and don't use their recommended UV lamp (other UV lamps may cure it just fine).
The main differences I've noticed between the two are:
1) iLac is easier to remove
2) iLac is a bit "rubberier" in feel when on the nail than Shellac (it reminds me a little of Biosculpture). I haven't had any lifting issues, but suspect that it may be more prone to lift than Shellac if it was touching the cuticles.
3) I think Shellac is a bit easier to apply. I think this is a combination of the brush shape and consistency. iLac seems a bit more prone to flooding the cuticles.
I have to say, the shine from iLAC's top coat is just as good, if not better than Shellac imo. They have a gorgeous vibrant purple that looks AMAZING with the iLac top coat. I've also played with iLAC and CND additives and that seemed to work ok as well. I do wish iLAC did a nice transparent pink for French as I don't really like the pink they recommend, it's too opaque for my taste.
I've also tried CND shellac VIP Silver Status over iLac colour and then used iLac top coat over the top and that seemed to work ok as well (NB: Before anyone says anything, I'd not do this on a client, I just tried it on my own nails out of curiosity).
If you are in doubt, the iLAC's such a good price that you can easily get a few colours just to try initially - that's what I did, and now I have about 30 of them!!
) I still really like Shellac too, but I think iLAC's a nice way to bump up your colour range fairly cheaply
Oh, one other thing - I have IBX on my nails too and iLAC seems to work fine in conjunction with it.