I just did some work in a
theatre which had Sennheiser G2 100 transmitters (handheld and bodypack) paired with a Lectrosonics receiver system. The system works wonderfully, and doesn't cost much more than an all Sennheiser system when one factors in the antenna splitters that would be needed and the like.
The Lectro VENUE system has six receiver modules in one rack space, and internal antenna distribution with a loop thru port. The only limitation is that the frame is limited to a 50 MHz bandwidth, including units connected to the thru port. Modules tune across 25 MHz of bandwidth or so. However, this is very much a benefit because the receivers will not be bombarded with strong out-of-band signals which might desensitize the receiver. Additionally, Lectro is known as one of the best wireless vendors out there for their quality.
One real benefit of going with the Sennheiser transmitters is that they're quite small and made of metal, not plastic like the Shure and AT units. Thus they're able to take a lot more abuse than their competitors. Plus, they don't cost too much by themselves.