The Raspberry Pi and Beaglebone product lines are still the single board computers to beat for tinkering and hobbyists in my non-expert opinion. Between those two, Raspberry Pis are better suited for HTPC roles.
Other SBCs have superior hardware but those two have the large, active communities which are indispensable for researching a project and troubleshooting. Equally important is that the companies and foundations behind the Pis and Beaglebones support their products. It's common practice for "open source" SoC manufacturers to release source code for only one Linux kernel version plus binary blob driver modules. Users eventually must chose between an EoL kernel no longer receiving patches or a current kernel without hardware drivers, if one is even available. This wiki page
for Allwinner SoCs is a good example. Boards based on Rockchip and Amlogic chips suffer from the same problem; the user communities need to pick up the slack or they become effectively abandoned products fairly soon after release. It's a shame because some of them have nice hardware for the cost. By contrast, official disk images for Pis and Beaglebones are regularly released with updated kernels and software packages.
Is $40 even a good price for something like this?
The $35 or whatever board prices are a bit deceptive because they will probably need at least a few accessories. A power plug will be several additional dollars because your phone charger may not provide enough voltage and amperage to keep the board and peripherals(like a wireless adapter) happy, a case will be another several bucks, maybe a new SD card also, and so on.
Are these dev boards up to a point where they're worth using?
As light duty servers, network appliances, media streamers, or brains for a hardware project, sure. As general purpose computers or HTPCs that are expected to transcode on the fly, entry level x86 machines are probably better bargains despite higher cost, and they have the option for Windows should the open source operating systems not work out. There are x86 SBCs
but I personally would rather have a low end NUC or mini-ITX system for the price.