I used them in past, well yes very cheap, but 99% of them are garbage. Also, I haven't noticed any improvements on my serps.
I've not bought from them, but I've heard many reviews like this one. Avoid backlinks.com
I noticed it can take some time for visible changes. Even if all links already indexed it can take one week or more. But sometimes (very rare) I see immediate changes in SERPs.I have baught some high pr backlinks that I am testing on a few new sites. Keeping a close eye on SERPS, no improvements yet, waiting for the links to get indexed.