Yes you're right, it is a term of endearment. It is somewhat old-fashioned but hard to say. I rarely use it, unless to be funny or when talking to my sister ;-) But some of my friends use "Schatzi" or "Schatz" pretty often refering to their boyfriends.
It may be dated in some regions but it certainly isn't in Austria - it's widely used as a term of endearment here.
Hutschi, at least how the term is used in Austria it is not really equivalent to English "dear": the English use "dear" also for persons which aren't particularly close to them, however "Schatzi" always indicates that you're quite close to this person, in fact it usually means that the person referred to is either your partner or else a very close friend (only in ironic use this might not be the case).
While you can say "Du bist ein Schatz!" to every person which did you a favour, or was nice or whatever, the appellative "Schatzi" is much more personal, and certainly more personal than English "dear", at least - as said - the way it's used here in Austria.
in german parts of Switzerland its used also at work to create a familiar atmosphere.but more between women.its not really in a positive sense.depends how it is pronounce and the context its used.then its more like Dear in english.F ex Dear u forgot to clean the table.Schatz,du hast vergessen den Tisch zu putzen.
-chen and -lein are diminuitives which can show affection. -i shows affection and can, additionally, express a kind of diminuitive.
Schatz exists in all variants: Schatz, Schätzchen, Schätzlein, Schatzi, Schatzilein and all kind of dialectal forms: Schätzle, Schätzla, Schatzel, Schatzerl (= Southern variants - perhaps someone can add the Northern ones ...)
If a comparison has to be made with English expressions, I would suggest that 'Schatzi' is similar to honey, the way this word is/was used especially in American English.
Ich lieb dich so sehr, Schatzi / I love you so much, (my) honey.
"Honey" found its way into German language, too, but is seldom now.
In German I heard also "Honigschnäuzchen" - also with a diminutive "Schnäuzchen" (If I remember right even in a movie with Louis De Funes.)