I've had the saline ultrasound, and if anything, it's just plain messy! It doesn't hurt at all. I was a wreck when I had it done last year and it was a piece of cake.
The doctor just inserts through a catheter some saline to show up polyps and fibroids better on the ultrasound. Only takes a minute and he's done, at least my doc was. It just makes a mess when the saline comes back out but it's to be expected. It doesn't hurt at all, maybe just a little cramp.
If you have a polyp, though, it's best it comes out because you don't want those growing in you. I have a large one plus a large fibroid and I've had it with the D&C route because I just keep producing them all over again in no time, so that's why I'm having a hysterectomy.
Best of luck!