A company I do business with as an external contractor wants to pay me via ACH from here on in. It’s optional, though, so I thought I’d ask y’all what you thought. Any particular pros or cons that most people wouldn’t think of?
Pro: you will finally enter the 20th century.
Con: WTF is ACH?
Automated Clearing House. If you’re a regular employee somewhere, it will be familiar to you as Direct Deposit.
Automated Clearing House (Wiki article).
ETA: Ninjaed by friedo. The wiki article described (briefly) how the information flows in several scenarios, including a scenario where a payment fails due to insufficient funds.
ETA2: Since OP asked for pros and cons, here one that I (as a total laydoofus on this) wonder about: It sounds like the payor gives consent to have his check converted to an electronic format and submitted to his bank electronically, or something like that. More and more, when you pay something by check or on-line, there’s some disclaimer that you’ve implicitly agreed to this. This makes me wonder if I’m giving somebody authorization to pull money out of my account without having an actual paper document (the check) signed by me, and if this can be abused or otherwise result in erroneous debits to my account that I didn’t authorize. ???
In the case where you’re paying with a check that gets converted to ACH, the check serves as written authorization to take money from your account for that transaction. It can certainly be abused; a dishonest merchant can enter the wrong amount or enter several transactions into his system. But that’s equally true with paper checks as well.
There is also the case of direct deposit and automated utility payments, in which case you sign a thing giving permission to credit or debit your account on a regular basis. There are no checks involved in those cases.
For some companies it is the only way to get paid. The previous place I worked was like this. You could receive a “live” check for only the first 2 pay periods, so you had time to set up a bank account if needed.
After that you had better have a direct deposit set up or your pay would be held until you did.
It’s the only way I will get paid again, live checks are a hassle. Can’t remember the last time I went to my bank in person. Don’t bother getting a monthy bank statement anymore either. I don’t even bother picking up the pay stub anymore. My employer keeps them for awhile then shreads them.
I just log in to my bank account on line and there is the money. Don’t keep a check register to record checks I have written either, they show up right on line too. All debit transactions are there before I even return from shopping. Pay my mortgage, Visa, and other bills with a click of the mouse too, transfer money between accounts.
When direct deposit first appeared, I didn’t trust it and would have refused it if my employer had offered it. Later on, I got to thinking about how nice it would be to not have to take my paycheck to the bank every week, using up most of my lunch hour in the process. Eventually, when my employer started offering DD, I took it, and I’ve never regretted it. They still give you a statement, with the usual information, but with the words “THIS IS NOT A CHECK” written across the bottom of the statement (where the check would normally be). I call it my non-check 
I just look at the non-check to make sure everything is OK, and then I file it away when I get home. No more running to the bank to deposit anything. I just swing by the ATM when I need some cash. On the odd occasion when I’ve gotten a rebate check or a check to cover a personal debt, I deposit it in the ATM as well. I haven’t been inside a bank in months, maybe a year or more.
I don’t understand people who act like they never have problems with it. We always have. The biggest thing is that you don’t know when it will hit. Sure, it seems like there’s a pattern, but then, all of the sudden, one time it will be different. You’d think that having it done through a computer would mean it would be more regular, but it hasn’t been in my experience.
Another problem is just that it’s much easier to forget about something when it happens automatically. I guess that’s not a big deal with deposits, but it’s always come back to bite me with automatic payments.
I’m sure that, over time, I’d get used to both of those things (and I’m going to have to, it’s being forced on me, too–even though, up to now, I’ve lived without a bank account for years. I hate dealing with banks.) but the idea that none of you ever had problems with that seems so weird to me. Even if you can afford to have a buffer in your bank account, I’d think that you’d occasionally mess up and have to use that buffer.