The clunky plastic things on your feet. They are clunky to ensure that the force and position of your leg is transferred precisely to your skis so you can turn when and where you want.
Depending on how well they fit, boots (regardless of brands) can either be torture devices or so bloody comfortable that you forget you are wearing them. To ensure comfort, you should spend a bit of time with a good bootfitter who will look at your feet and put you into boots that suit your foot shape and ability level. Different brands of boot, and models within brands, are made on different lasts and suit different foot shapes. The only thing that matters is the fit. Whether or not your mate likes a particular brand is irrelevant, because you will have a different foot shape. Be honest with your bootfitter. The bootfitter could not give a rats about how good a skier you are, and just wants to do a professional job and put you into the right boots for you. They also have a financial interest in not having you come back under their fit guarantee. Adjusting boots is an expense.
Most reputable ski shops will offer a fit guarantee which allows you to return to the shop an unlimited number of times for adjustment until the boot is comfortable. Comfort in the shop does not necessarily mean comfort on the hill. For example, most bootfitters will put you into a boot of a size that your big toe is touching the inside of the boot. This seems weird, but on the hill the boots will be comfortable. The difference in comfort is in part because the shop is nice and warm thus causing your foot to expand a bit, and in part because of the difficulty in precise repilcation of a skiing stance in the shop. While on the actual ski slopes, it's wet and cold thus making your feet shrink in size. Trust your bootfitter here, but if they are uncomfortable when you ski take them back to get them fixed. A good bootfitter has an arsenal of tweaks and fiddles to make boots comfortable, including packing, padding and, in extreme cases, blowing the shell out. Notwithstanding the availability of the tweaks it is better if they are not necessary.
There is a strong argument to buy on snow rather than a shop off snow, but only if the shop has a decent range of boots to choose from and has an experienced fitter. The key to this is still to find a good boot fitter and that you are there for at least a week. This way you can get the boot fitted and adjusted to your foot then go out and ski in it and take it back a day or two later and get more adjustments made to it if necessary. Any shop that doesn't offer this free should be avoided. Boots like all shoes tend to pack down and wear in a bit so how they feel on day 0 (in the shop) and after day 3 can be different and if you are on snow you can get this sorted then and there rather than skiing in pain for a week and then going back into a shop later and maybe not going back out in your boots again for weeks (or months!). A weekend isn't really long enough to achieve this.
One, apparently counter-intuitive, tip is that if your shins hurt try tightening your top two buckles. Often sore shins are caused by boots rubbing against the shin. If the buckles are tight, there is less rubbing. Another tip is to make sure your bottom 2 buckles over the arch of your foot and toe are so loose they almost come off. These are only there to ensure the boot holds its basic shape and are easier (possible) to get on. If you tighten them you will pinch the nerve to your toes and this results in numb toes. If a boot is too narrow across the area just behind the toes known as the metatarsal heads you will feel cramps and eventually a calcium build up in the form of a painful lump will appear on the metatarsal heads.
One thing that is almost always worth having is customised footbeds - individually shaped foot supports for under your feet. These improve both comfort and the boot performance and cannot be recommended highly enough.
Boot manufacturers' marketing blurb usually has a slew of pseudo-scientific mumbo jumbo with lots of numbers and indices and flex ratings and graphs and stuff. This stuff is generally meaningless. Unlike with bindings, there is no international standard for boot performance. The numbers are possibly useful for comparison within a manufacturer's range, but useless for comparing boots from different manufacturers.
If you are concerned about whether a particular brand of alpine boot is compatible with a particular binding, relax. The boot/binding interface is stanndardised and all boots will be compatible with all bindings.
Cross Country Boots
I am aware of only 3 brands that make tele boots. They have differing ideas on the shape of a human foot so you need to try on more than one style and brand. Teleboots are softer and more comfortable to wear than Alpine boots. Some people complain about the duckbill at the toe, its never hindered me.
I've never tried them, can be hard to get.
Links to Boot Manufacturer Web Sites Although there are many brands there are only a small number of manufacturers as there has been significant consolidation in the industry over the last couple of decades. This is true for most of the ski industry.