A friend of mine is interested in tandems and basically knows it's about two people on one bike. He saw a new tandem for sale for $300 and had no idea if the low price means it's junk. My bet is that it is.
If he and his potential stoker are regular recreational cyclists, then my advice would be to look out for a good second hand tandem (unfortunalty this will generally still be quite a bit more than $300).
If they are reasonably new to cycling, want to try out tandeming without too much of an investment, then a $300 tandem might be a good start. If they enjoy it, I suspect a much better bike will be on the cards in short time.
To me, the $300 tandems I have seen are good for relaxed cycling on a "flat" stretch for maybe 10-15 miles at a time. Don't expect any kind of long term reliability if they are going to be clocking up 80+ miles a weekend. Kind of reminds me of the "mountain bikes" with the warning labels that say "not to be used off road".
The way I see it - if they have good single bikes, a $300 tandem will be a turnoff (handling, weight, performance etc.), however if they are new to cycling, they won't know any different and a low end tandem might be a start to a wonderful experience.
Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
I think that most of the difference is in the bike frames.
When I owned my shop I used to keep 3 tandems in stock: an entry level Univega or KHS, a Burley and a Santana. It was interesting to test ride one right after another. Riding the low end KHS right after getting off of the Santana felt so flexy that it was scary.