After doing both FOH and monitors for years, I have noticed one big thing with in ear rigs. Almost everyone uses Shure transmitters and receivers unless you are willing to spend more than $1000 pre channel. In the last year or two, there have been some other additions to the market, but most people that switched, seemed to have gone back to the Shures.
First, the Shures are very durable. Depending on which model you get, they may not have as many options, but the Shure frequency locking seems to be more solid. They also have a MUCH better range. They are easy to set up, easy to program, solid during performance, and durable (except
the PSM 400's which have plastic packs, so be a little more careful with those). My second choice would definately be the Sennheisers, but I know a lot of people who get interference more often with those and cutouts, even though they do offer more frequency options.
The other thing I have noticed is that most bands carrying their own in ear racks do not use the Shure ear buds. For me the E3's and E5's are OK, but the equivalent Futuresonics, Westones, and Ultimate Ears seem to handle volume better, be a little bit more comfortable, and just plain sound a little better.
As far as stereo in ear mixing goes, it certainly can be distracting. However, 9 out of 10 times this is due to bad monitor mixing. For some reason people seem to think that stereo some how means hard panning. There is no reason you should HAVE to hard pan guitars, or overheads etc.... If the ear mix is too wide it will just seem very unnatural and I can certainly see how that would be very distracting. Try narrowing you hard panned items down a bit and all of a sudden the ear mix sounds much better. I have also noticed that with a little bit of imaging in an ear mix you get much better clarity of ALL the instruments and stuff in the mix, and that artists seem to be happier with a lower overall volume in their ears than when mixed in mono. They also seem to be less prone to needing changes from song to song. By lowering the volume through stereo imaging and proper placement, this also seems to help the bands "live vibe". The ears become more natural feeling and it lets you focus on your playing as opposed to your in ear mix. You can almost forget that you are wearing them.
Custom molds are also a wonderful thing for in ears. First, thay block things out better which allows you to get a tighter better mix at lower volumes (saves your hearing more), they stay in your ear better, and they almost always actually make the frequency response and over all tone of the in ear monitors better. I highly reccomend getting custom molds for all in ear users that can possibly do it.
In the end however, in ears are not for everybody. Some people just never seem to adjust to it very well and just never will. I have noticed though that a lot of groups are kind of afraid to use in ear monitors. This is almost always due to a bad experience somewhere. I can not tell you how many artists I have worked with that now love in ears because they finally got the right mix and the right setup for them. Mixing monitors for in ears is very different than mixing monitors for wedges. Wedges is often about control, volume, and logisitcal management, and certain sacrifices due to room interaction, and wedge to monitor interaction. Wedges won't always be perfect because there is often just too mnay factors working against them. In ears can be perfect every time if the proper setup and adjustments are made. 90% of the live sound logistic problems are solved by switching to in ear monitors. But still they aren't for everyone