The places I've played are as different from a casino as night and day, except that the game is played with cards, chips are used to wager, and most days a flush beats a straight. Most games in my area start up with a tournament and cash games fill up as the tourney combines tables. I like to just arrive late and avoid the pain that is tournament poker, and play the cash games as they get rolling.
When you sit down at a table in Vegas, the rake and min/max buy-ins are clearly posted, the dealers rotate in shifts, and there might be one or two regulars at the table who are friendly with the dealers. In an underground game there might be one or two people who aren't regulars, I've only once ever heard the rake explained, and there typically is no maximum buy-in.
Back up, revisit that. In an underground no-limit hold ‘em game, there typically is no cap on the buy-in. That makes a $1/2 game play much higher than in a casino, and also much, much looser. I've watched a player blow through multiple $500 buy-ins at a $1/2 game, before winning it all back over the course of three hands. I've seen $600 preflop all-ins get called in two places, and I've seen a guy make multiple $100 bets without ever looking at his cards. Oh wait, that was me. Never mind.
In an uncapped game you need to have a good understanding of your game. Some of my friends tend to amp up their monetary investment, buying in heavier than normal to be equivalent to the big stacks at the table. I tend to buy in for my normal $200, which gives me 100X the big blind, and am more likely to wait for a big hand and push rather than trying to extract maximum value. Frequently in these games the all-in is the way to extract maximum value, as you will almost always get at least one caller, no matter how big the bet.
Keeping an eye on the rake is tough in an underground game, as some places won't even tell you what it is. One room I play is 5% rake, no max. Another is no rake on the first $20 in the pot, then 10% up to $90, with a max of $9 rake. Yet another is nothing for the first $10, then 5% up to $5 max. Obviously I'm playing more speculative hands in the third place, because there I can win small pots and still beat the rake. At the other two places I'm looking to play bigger pots to beat the rake.
Dealers at underground games vary as widely in skill as the dealers in a casino, but if you get a bad one, you're usually stuck with them for a larger portion of your evening. Some places allow the dealer to play while they deal, which I'm not a fan of, as it can slow down the action, but with a good dealer they can play, deal and keep the game moving right along. There will be a lot of people who are friendly with each other and the dealers, and that can be intimidating on your first few visits. That's normal, just think of it as a bigger-than-usual home game, relax, and have a good time slinging cards with your new "friends." Then clean them out and head home.
Most places will still enforce all standard card room rules, particularly the English-only rule, and you still have the right to ask the dealer or the guys running the game if you think there might be something untoward going on. An underground room operates for the same reason a casino does - to make the operators money. They understand that if they allow rampant cheating or abuse of other players, that they will lose customers, and that's the last thing they want. Cheating, or the suspicion of cheating, is also a good way to end up with the cops on your front door, and nobody wants to see those fellows show up in the middle of a hand.
Of course in an underground game there are lots of other things to be wary of, like people following you to your car afterwards, but I've never heard any report of that happening at any of the games around here. But in general, if someone gets bad beat out of a huge pot and leaves steaming, it might not be a bad idea to rack up at that point and beat a hasty retreat before the boys in blue arrive to break up the fun.