Not all Winmodems are equally bad.
There are two kinds of Winmodems:
* controllerless. The host CPU handles protocol-related matters. The actual generation and translation of audio into a proper bitstream (including things like echo cancellation, noise suppression, etc.) is handled by a proper DSP. Modems of this type have the potential to be quite good, and in some ways are objectively superior to traditional modems. More on this in a moment.
* host-signal processing (HSP). These are evil. Bad. There's absolutely NOTHING good you can possibly say about them. These are the $6 modems that consist of little more than a tiny PCI card with a few connectors and a single (small) chip in the center. Unlike controllerless modems, these modems don't even have a digital signal processor to handle the audio to data conversion. They LITERALLY are nothing more than a minimal soundcard with a few low-tech parts to handle the impedance matching between the phoneline and sound chip. They digitize the modem audio in realtime, and rely upon the host PC's CPU to do the millions of fast fourrier transformations necessary to turn it back into a stream of binary data. This is a lot more work than you'd think, because modem audio is NOT a single sine wave. All kinds of behind the scenes digital signal processing needs to be done to turn an audio stream resembling random white noise into something resembling 56k data.
You've probably figured it out by now, but the use of a $7 HSP winmodem is the fastest way to squander half the market value of your blisteringly fast 2+GHz CPU (it'll soak up about 20% of your CPU speed when in use. Compare the current price of your CPU with the current price of one that's 20% slower). And don't even get me STARTED about how easy it is to f**k up the drivers for such a modem badly enough to require either replacing it with a new modem using a different chipset or reinstalling Windows.
HSP modems to avoid like the plague are ANYTHING with a PCtel chipset, the letters "HSP" on the box, or a PCI card with just a few components and a single tiny chip in the center. As far as I know, all of Connexant's chipsets are HSP, though it's not inconceivable that they might have one or two controllerless ones that use a DSP. I believe ALL CNR and AMR modems are HSP by definition.
Now, for the good news. CONTROLLERLESS modems entail very little sacrifice. The amount of CPU processing required to handle protocol-related stuff (responding to AT commands, data compression, etc.) is virtually nil. Unlike HSP, where the host CPU's undivided attention is required on a regular and frequent basis when connected, controllerless cards are entirely capable of buffering the incoming bitstream for a few milliseconds at a time when the CPU has something more important to worry about. Kind of like the difference between old CD-R drives that required a 150-600kbit/second datastream in absolute lockstep and would churn out a coaster if the CPU's attention wandered for even an instant, and new drives that can buffer the bitstream and tolerate periodic inattention from the CPU.
Everyone loves RS-232 serial modems because they're painless to connect, don't require drivers, and work with everything. Unfortunately, RS-232 is probably the LEAST efficient interface you could possibly use to connect anything to a modern computer. RS-232 is an old, slow, inefficient protocol. More importantly, because it's so slow, it's one of the "sloppiest" functions of a modern mobo chipset, rivaled only by the floppy drive. Few people realize that 16550 emulation is just that -- emulation. Not as in, "a virtual 16550 is embedded in the chip and offers the same functionality," but as in, "the chip lies and claims to be a 16550, but in reality it can only buffer a few bytes at a time before overflowing because the designers (or their bosses) knew modern CPUs are fast enough to hide that little sin anyway, so nobody will really notice the difference." That's why Windows (and presumably Linux) offer the option to ignore the fake 16550 entirely. As a practical matter, Windows and Linux are usually reduced to polled RS-232 I/O, just like they were 15-20 years ago.
On the other hand, a good controllerless winmodem (like the Lucent/Agere ones) can buffer a good chunk of the bitstream and periodically deliver it to the CPU with minimal overhead. In many cases, a system using a controllerless (but NOT HSP) Lucent-based winmodem performs BETTER than one with a modem connected to the db-9 serial port behind the computer, simply because the added protocol-related work is minimal relative to the hoops Windows/Linux needs to jump through to use the fake 16550-based serial port.
That's not to say someone like USR can't make a PCI card with a high-quality serial port that has all the advantages of a controllerless lucent winmodem... but then you've lost the advantage of not needing drivers. And as I said, the real overhead imposed by relying upon the host CPU to do PROTOCOL-related stuff in virtually nil. A good, controllerless Lucent/Agere-based modem (WITH DSP) costs about $20 online or at a computer show. The PCI USR modem with the quality serial port and 100% hardware-based modem costs about $100. Is it worth it? If your ONLY means of getting online is via modem, "maybe, but probably not." If you've got DSL/cable and only keep the modem around to send faxes and act as a safety net to keep you sane once or twice a year when there's an extended cable/dsl outage, I'd say "no way in hell."
* Lucent/Agere explicitly supports Linux, and apparently their Linmodem drivers work quite well. They're NOT open source. Whether or not you care is a matter of personal ideology for you alone to decide.
* Lucent's modem chipsets are now manufactured by Agere. I don't know whether Agere is a new division, someone who just bought it, or what, but the chips themselves are the same.
* Not ALL winmodems with a Lucent/Agere chipset are controllerless and have a DSP. They DO have at least one chipset that's HSP. So make sure you do your homework before buying, especially since the retail price difference between a controllerless and HSP Lucent/Agere winmodem is a whopping $5 or so.
* I'd recommend against HSP from anyone, even Lucent, but if you've got no other choice, Lucent HSP is STILL way better than PCtel, and I'd trust it before Connexant and Cirrus too. But NOTHING is worse than PCtel.
* If you're feeling really evil, buy a PCtel modem and give it to someone you REALLY HATE as a gift. Then cackle with glee, knowing that they'll ultimately have their computer slowed down, and probably burn several days at some point trying to make it work when something upsets Windows and it decides to stop acknowledging its existence ;-)