There are many different types of UPS - and as mentioned, many are designed to be just "good enough" to keep the computer running so you do have to be careful. Most of the UPS's are line-interactive, which means that they just pass the line voltage through as long as it is within range. Some UPS have power conditioners and filters built in, so the power is being filtered by those circuits until the voltage drops too low (or too high), then it switches to the battery and inverter to make the power. These are less expensive and make the batteries last longer but do not generate the output all of the time. The In-line UPS is constantly running the inverter and drawing its power from the battery, while another circuit recharges the battery. These are usually very expensive and are fairly rare, since most are the line-interactive design. I believe all in-line UPS do output a true sine wave.
Of the line-interactive type, many of the cheaper ones do not output a true sine wave. Instead, the inverter outputs a square wave, triangle wave, or stepped square wave to approximate the same RMS output power, but it is a very noisy wave, and would likely damage audio equipment, or at least make components buzz etc. The computer power supplies aren't as sensitive so they can handle that type of wave. APC's SmartUPS series outputs a true sine wave so you could use that with audio equipment. The SmartUPS also have a power conditioner built in to filter and keep the voltage within range. If the voltage goes a bit low, like 100 volts, the power conditioner's auto-transformer brings the voltage back up to 120 without using the battery. If the voltage drops below the range of the auto-transformer, it will switch to battery power. Also when the unit senses voltage surges or spikes, it will switch to battery power. The APC is a good choice for protection especially if you have bad power problems in your area, it gives you some confidence to keep your equipment on, but I'm not sure how well it does as a filter/conditioner compared to the ones designed specifically for audio. I suspect that since noise filtration is not their primary concern, that the UPS deals more with protection and always-available power where the conditioners with audio in mind focus on noise filtration.
