The GMAT Club tests pleasantly surprised me. The quality of the quant questions are top notch (aided no doubt by the user submitted questions) and the analytics are great as well. There is a in built error log that probably needs more work, but it is more than enough for basic use.
The verbal section, though, needs work. OG questions are there, but the selection is largely muddied by the user submitted questions that are generally much worse in quality. I steered clear of the verbal section entirely.
Overall, this is a top notch product for Quants and I'd highly recommend this to any prospective GMAT student.