When I build up a new bike, I spec out a bunch of the non-group parts in a specific way:
I want to install it and forget it.
What I don't want is a part that will loosen up spontaneously, or be difficult to adjust, or crack after 6 months of summer abuse.
At the same time, when people ask me about setting up their own bike, I tell them to get good non-group parts. Wheels, of course, but that's worth twenty posts itself. The other parts include the saddle, pedals, bars, stem, and seat post.
Each of these bits play a critical part on your bike - if one breaks, or loosens, or gets compromised in any way, your race is over.
Other parts may not be critical, even if at first glance they seem "critical". My good friend and long-time co-promoter Gene broke his rear derailleur cable in a critical race (he was working for me in the race). His bike suddenly became a two speed bike - 39x12 and 53x12. Nonetheless he contributed a lot to what ended up being a close win for me. He even launched a last lap attack to string out the field, per my request before the race.
In a more high caliber race, Tom Prehn (former winner of the Philly race, a long time domestic pro, and now the boss at Cateye) started off a race with a broken front derailleur cable or clamp. Whatever it was, he was out of luck, with no spare bike, at the start of a 100 mile road race in New England. He quickly realized that he wouldn't make it to the big climb halfway into the race if he has just his small ring, so he quickly maxed out his limit screw to keep the chain on the big ring.
Halfway into the race, on the big climb, he went to the front, rode in the big ring (of course), and pushed the pace. Everyone else, content to let him set the pace, simply followed him. At the end of the day he won the race.
The point is that if you can still ride or pedal your bike, you can still race.
But if your bars break, or your stem doesn't stay tight anymore, your seat rails disintegrate, or your seatpost drops into the seat tube, you're SOL.
You know, for "So Outta Luck".
A part is working well when you no longer think about it. Because of that I've been using Thomson seat posts since, well, about 9 or so years ago.
Seatposts, according to my "lose weight in the hidden, boring components" rule, end up a prime place for bicycle weight loss. You'd be surprised at how much some posts weigh. Light posts let the bike wiggle a bit better when you're sprinting out of the saddle. They can remove a good half pound off of some bikes, more on lower end ones.
I let myself get seduced by superlight posts back in the day. You could tell that my (younger) glazed eyes ignored various warning signs. I ended up losing my saddle a few times when bolts or clamps failed, one post start moving around when the post got crushed when I hit a bump (I was all of 140 lbs or so), and the wide assortment of parts I've saved from various different seatposts.
At some point, when I got sick of my posts moving, sliding, twisting, crushing, cracking, and snapping, I went looking for the lightest super reliable post possible. I ended up with two final choices, but one company's product has been reliably available through various shops:
When I put my Cannondale together, one of the most important things I did was to install a Thomson post. Likewise, when I realized I would be racing the Riggio track bike more often, I put one on that too. Even the Giants I had before had Thomson posts.
So, yeah, I like the posts.
They're reasonably light. They adjust easily, but don't move once you tighten down the adjusting bolts. I can forget about it once I'm done with it.
In other words, they work.
That's all you can ask from a post.