I've used knock-offs before and there's a reason they're knock-offs. I have not had good experiences with them, at all. The only exception is Mega-blocks, but they're not knock-offs really, they're just as pricey as LEGO, they're just an alternate LEGO-like creator.
There's a reason LEGO (and Mega-bloks) sets are as expensive as they are. The quality control, consistency and precision of each and every piece is an engineering accomplishment. The pieces from sets you bought 20 years ago will work with and fit snuggly and securely on pieces from sets you buy tomorrow.
The same cannot be said for the knock-offs. In all of the sets I tried (3-4), the pieces were not consistent, one set I couldn't even complete because the pieces didn't fit together right and on one of the sets it literally flew apart on it's own halfway through because the pieces fit TOO snuggly and the tension caused the pieces to eventually pop out of all the connections.
If you're at all serious about the sets and having them last, not to mention even be able to be put together, do not buy knock-offs. The risk of getting a set you simply can't even use is just WAY too high in my experience.