Be aware of subpar adapters! You'd think that if an adapter were sold to mount a given lens to your DSLR, it would be made with the correct flange back distance to do that. With the better adapters, this is the case. With some "bargain" adapters, however, quality control is such that a specific example actually may be a bit too long or too short. Some adapters contain glass elements to allow infinity focus when the flange back distance of the lens/adapter/camera combo is too long, but these extra elements often reduce image quality, and they cause the adapter to function as a short teleconverter, increasing the effective focal length of the lens—not optimal for wide-angle work. It's best to get an adapter of the proper thickness for the camera body in question. And, thus, it's best to buy your adapter(s) from a source that will let you return any that don't provide proper focusing.
Another consideration is lenses that protrude into the camera body. If the lens you wish to use on your DSLR protrudes into the camera body, it could interfere with the SLR mirror operation. Such lenses shouldn't be used with DSLRs (they could be used in Live View mode with the mirror locked "up," but damage could occur if you or the camera lowers the mirror with the lens attached). Definitely always check with the manufacturer of the adapter you intend to use as to whether a specific lens can be used with a specific camera body using that adapter (and sort of "eyeball" it yourself before attaching the lens, just to be safe).
Stopped-Down Metering And FocusingWhen you use a DSLR with a lens that was designed for it, everything is automatic. When you select an aperture for a shot, the lens aperture remains wide open for easier composing and focus-monitoring, then stops down to the selected value when you fully depress the shutter button to make the shot.
When you use a lens adapter, the linkage between camera body and lens is broken. The lens will remain at whatever aperture you set. So you'll have to manually set the widest aperture to get the brightest possible viewfinder image for composing and focusing, then manually set the desired aperture for the shot. This is inconvenient compared to auto-aperture operation, but not a big deal with nonmoving subjects. It's a bigger deal for action subjects. Back in the day, sports shooters generally worked wide open, in part, to get a fast, action-stopping shutter speed, in part, to have a bright viewfinder image to work with and also to avoid fooling with the aperture ring while shooting. One more thing to keep in mind is that some lenses change focus when stopped down. If your does, you'll have to focus at the shooting aperture, even though that provides a darker image and greater depth of field, making focusing more difficult.