Ugh, coordinating a group rate was BY FAR the most frustrating and stressful part of the entire wedding planning process...rest assured, once you do that though it should be smooth sailing.
We went with a TA and it doesn't necessarily make it that much less stressful and we actually have a phenominal travel agent. I think part our hassel was that we needed rates for three different gateway cities. Anyway, in the end our TA found us a great deal price wise and nothing on the online discount sites has been able to compare...however, the catch is that because it's an early booking bonus everyone has to pay in full by October (the wedding isn't until April 2011!!). In hindsight, if I had to do it all over again, I'd be really, really tempted to just have all my guests fend for themselves and book however they wanna book...some people will likely do that anyway.