With bell boots, I often think simpler is better. 

 For the most part, I prefer the old-fashioned pull-on gum rubber bell boots. They are sturdy and stand up to a lot of use. Yes, they can be a bit of a hassle to pull on and off, but they are light weight and flexible, and you can keep them supple by rubbing them with baby oil now and then and letting it soak in overnight. They're also quite cheap :) They do move around a bit, but my mare has had to wear them 24/7 for weeks at a time (she was pulling shoes in the paddock for awhile when I first had to start shoing her) and she never got rubs with them, and she's young and active. 

 If ease of use is important, several manufacturers make rubber bell boots with velcro tabs. In particular I've had good luck with the Roma brand ones. They're not expensive and they hold up well. They even have a ones with synthetic fleece around the to, and they come in cute colors as well as traditional colors.

 I've had Davis and Professional's Choice bell boots as well, and didn't care for them. The Davis ones rubbed my mare's pasterns and the  Pro Choice ones which are designed not to move/spin caused a pressure point - they use a ball of material that fits behind the fetlock into the groove above the heel bulbs). The Pro Choice ones were also quite heavy when wet and took a long time to dry out. 

 I have tried about every kind of bell boot out there.  The only ones worth spending money on (don't rub and stay where they are supposed to) IME are (1) the Italian KL gum ones and (2) the petal bell boots (VTO and Bit of Britain sell them).  Everything else rubs, falls apart, doesn't sit right, doesn't cover, spins when it's not supposed to or otherwise failed to satisfy.

Emolee:

I've always been told to use a protective boot (splint or otherwise) on my horse when I work him. To be completely honest I never questioned it, it was always just something I did without thinking. If I'm doing flat-work in a tilled arena, do I even NEED to use them? I'm reading conflicting articles and sites and it sounds like it just comes down to opinion, but I'd like to hear your guys's personal perspectives.

I think it really depends on both the horse and the work being done. I ride an older QH who has really wonky legs. He needs the splint boots. Even just walking out to the arena, I can here them rubbing against each other - and if I don't put them on tight enough, they are are turned around by the time we are done. The little kids who ride him (on a leadline) in camp, don't use the boots.

If a horse is overstriding enough that s/he could pull a shoe, better to wear bell boots and prevent it. Both during work and turnout.

My horse is a baby (will be 5 this year), has big feet (draft cross w/ the feet of a full draft, lol), and just got his very first pair of shoes (fronts only) in Nov. So he is very likely to knock himself in the leg w/ another hoof - and wears splint boots when we are working. Not that we do all that much - 40 minutes of walking and 5 of trotting gets him very sweaty still.

 I agree that if your horse isn't forging or otherwise hurting himself, the boots are probably not needed.

I used to use the Sports Medicine Boots on my horse because I assumed they gave needed support to a tendon he had bowed when he was younger.  I'm not sure they were necessary though -- he's 33 now and still runs round the pasture at full tilt, without any leg protection at all.

 You do not need to use boots for arena work unless your horse interferes and is hitting himself.  No boot provides any meaningful support to the leg, they just give a bit of impact protection.  When in doubt, I leave the boots off -- now I only use them for XC and I do put tendon boots on front for my green TB who is learning to jump, that's it.