I have been using Mountainsmith Day and Kifaru Tailgunner II lumbar packs for years - for everything from day hikes and environmental field work to bicycle commuting and family vacations. I am a huge fan of well-made, spine-hugging, hands-free lumbar packs but like most things, they do have their drawbacks. Some observations:
(1) Lumbar packs can hold a surprising amount of bulk comfortably, but not a lot of weight. So when considering a higher volume pack (like the Day or Tailgunner), recognize that it will hold your fleece, poncho, and ultralight tarp/fly just fine - but loading it down with extra water, tools, or magazines will dramatically reduce its stability, fit, and comfort.
(2) Like someone above already mentioned, you can attach strapettes or shoulder straps to improve the stability and load bearing capacity of a lumbar pack - but at that point you're usually better off just going with a traditional ruck. A regular backpack will carry and distribute loads of this size better.
(3) Good lumber packs have features like delta straps that help snug it down into your lower back, which greatly improves the balance and security of the load. They're absolutely essential, in my opinion. Anything else is a just a fanny pack. On the other hand, having to undo and redo the straps everytime you need to take the bag off is a pain. There's no way to just drop one shoulder and swing the pack around to access your stuff - you have to loosen the belt or take a lumbar pack off all the way to get at your gear.
(4) In terms of outdoor travel, lumbar packs are great for times when you might need to use your hands to hold on or carry things. They don't necessarily provide an advantage over backpacks in this case, except perhaps that it may be easier to still carry gear with shoulder straps (rifle, heavy SLR camera, etc) that would otherwise interfere with the backpack.
(5) For urban travel, backpacks are much better for getting in and out of taxis or buses. As mentioned above, it's more of a pain to take a good-fitting lumbar pack on and off for quick trips around town - and they're certainly no fun to sit on. If you're going to be removing the pack frequently, or accessing your gear regularly, you're probably better off with a backpack under these circumstances.
(6) Security is just as much of a concern for lumbar packs as regular backpacks. Despite being somewhat smaller and riding lower on your back, they are just as susceptible to pickpockets - especially as you're bumping through a crowd. A lot of people think their stuff is closer and safer and harder to take, but they're just not giving the thieves the proper respect. Having your $400 digital camera (and all your family pics) taken from the pack on your back while getting off the cruise ship can really take some of the fun out of your vacation. So take care and keep your vaulable stuff in deep, internal, and hard-to reach pockets.
(7) You will get just as sweaty wearing a lumbar pack in summer - more so, in one sense, because you always have to keep the belt strapped snugly to keep the pack on. So from your belly to your butt, it's gonna be wet. One thing that works for me is to take my shirt off but leave it tucked under the pack belt, and just hanging back over the pack. This way, you stay cooler and the belt/pack doesn't get too sweaty - and best of all, no slimy shoulder straps like with a backpack!
So my reccommendation is first to decide how much gear you have, how heavy it will all be, and where you're going with it - and that should give you a better idea of what kind of pack will work for you.