Those work pretty well, as do simple right angle squares and clamps. I have some. I'll probably use them today to weld up the mounts and levers for the retractable casters.
I have 5 or 6 magnetic right angle squares now. I'll probably use them today, but a jig table is so much faster.
Cheap magnets do have some issues.
They have almost no "clamping" force. This means if you want to constrain a weld out (there is more to it than that) they will not hold the pieces. You can use them to do a tack up, but they are useless to constrain a piece for a weld out. Of course welding choices (where and in what order you apply heat) and a hammer are a bigger factor as to how your metal distorts, but the magnets are useless to constrain the parts for more than a quick tack up
They also loose their magnetism with heat. Not that the metal becomes non-magnetic, but that the magnet itself looses it magnetism permanently. It does that at a much lower temperature than you would think too. Only around 300F degrees. Some at much less. Some special made Neodymium mags tolerate a little more heat, but not a huge amount. I use magnets in aluminum lead casting molds to hold inserted steel parts, so I am sensitive to this issue.
A jig table is faster and consistency is very easy. Especially if you are making more than one of the same part. It can distort from heat, but only in such that steel grows about .001" per 1" per 100F. You can square up two pieces very quickly and clamp them in place anywhere on the table just as quickly. It doesn't lose its hold from low heat, and you can clamp and unclamp parts without loosing their fit up.
I find myself welding more often and my knees less and less tolerant of working on the floor these days. If I am building a welding table I'm going to do it so it saves me the most amount of time and work on future projects. The nice thing about a grid of holes on the top is I don't have to do it "today." Today I'll probably throw a piece of plate on top loose, and get the lifting levers done for the casters.