I have two HomePods which I use with my iMac. Having one HomePod is fairly good as a desktop speaker but there are some drawbacks. I don't have many problems with lag when playing Music. There's often a bit of lag when you begin streaming music i.e. it doesn't begin playing right away, but once it starts playing there's no problem. With internet videos it's generally fine; I don't have many problems with watching Youtube, though sometimes it feels that there's just a slight delay with what's happening on screen and the audio, though it's extremely minor and unless I'm looking for it, its not highly noticeable. It sounds very good for Youtube videos as well. The problem with one HomePod however is with watching movies and playing games. I find the sound with movies to be significantly muffled and the bass very boomy -- I can't watch movies with one HomePod. Also when playing games, the sound can be very laggy, taking up to one or two or even more seconds to register what's happening on the screen which is essentially unusable for some games.
Now with two HomePods in stereo, the sound in movies is really good -- it's louder, the voices are clearer and the background sounds also seem better. And there's also no lag. But this leads to the next problem. You can only use stereo HomePods with iTunes when using a Mac. You cannot use stereo HomePods for general use on a Mac e.g. when playing movies on Youtube or streaming a radio station from the internet browser or for playing games. This means that stereo HomePods on a Mac can only be used for movies, tv shows and music if you use iTunes. This doesn't bother me because I watch all my movies from iTunes and I'm an Apple Music subscriber. But it's a significant limitation. So, one HomePod as a desktop speaker is very good for streaming music and quite good for watching general videos on the internet. But if watching movies on your computer or playing games is important to you, then I would be hesitant about purchasing it because it's not very good for those things (though there isn't any lag when watching movies). If you intend on getting two in stereo, then just remember that they're limited to iTunes.