I was in the same boat a short while ago as my first Final Fantasy was also X. And I have now played and beat VI-IX as well, and I'll someday get around to beating the other ones as well. Strangly enough though, my second FF was FFVI so that's quite a leap backwards. But, I liked FFVI even more than I did X.

As for you, I recommend that you try VI and if you like it, great but if you don't, no need to worry. If you can't get into it, then maybe it's just too much of a leap backwards for you. If this is the case, leave it alone for a while and try VII. If you like this, once you're finished with it, then try VI again and you may enjoy it now that you are a bit more familiar with FF games and it's not such a leap back this time. However, if you don't like VII as well, then I would be inclined to say that perhaps you just don't like the older FFs.

If this is the case, and you really like FFX and the setting, then I would suggest that you purchase X-2. Either that or start counting the days to FFXII.