I've been an eFax member for a long time, and although the service is great (I started with the free one, I now have the pay version so I can send faxes), I do have a couple of complaints. First, the delay was not 100% the BWC, there is usually a good 5 to 10 minute delay in the time from when a fax is sent to the time it is delivered via email (can me MUCH more if it's a huge fax).
Secondly, the faxes are delivered in a proprietary .efx format that can only be viewed using the eFax/J2 desktop software, which is a xxxxx if you a) want to view it on your BB and b) if you want to forward it on to somebody else.
If you can deal with those drawbacks, the fact that you can get a free fax number, it's a pretty cool service. Depending on which CSR you get when you call, you can also get a free voicemail box at the same number as your fax number - yet another cool feature, all for free!