With a 10-20, there are a couple of potential architectural issues. You'll have distortion but it's not pincushion and not barrel but moustache. In other words if you shoot a seascape with a horizon through the middle of the frame, it will be wavy.
It's slight, but it's noticeable. Certainly a lot more noticeable than another wide angle that has this form of distortion (the Carl Zeiss 21mm)
Similarly, when you shoot architecture you will notice slight waviness in your vertical lines. It can't be corrected in Photoshop, where barrel and pincushion distortion can.
Now if you can get over that, which I did for a couple of years, you've got a good value ultra wide angle on your hands. Sigma have two options - the f3.5 and the variable aperture. I'd go for the variable aperture as a) it's cheaper, b) it's better and c) it has a 77mm filter thread. This makes filters potentially a lot cheaper.
When shooting with any lens you'll have perspective distortion unless the lens is perfectly level (lens not pointing slightly up or down). For example, when angling the lens upwards slightly and shooting buildings the taller parts of the buildings will lean in from the edge of the frame. With an ultra wide angle this effect will be more extreme. Sometimes this can be used for effect but there will be times when you'll want to correct things in Photoshop. If so, always frame wider so you have room to correct the lean in photoshop. Preferably shoot with the lens parallel to the ground and then crop the wasted foreground in photoshop.
If you're shooting at eye level you will have a lot of wasted foreground - it's always quite useful to get down low and add an element of foreground interest into your shot as otherwise you will have nothing and then a very wide and small view of subjects in the distance. Sure, 18mm is wide enough in most instances, but the difference between 18mm and 15mm is big, then 15 to 10mm is another huge step. It will really add depth to your photos. If shooting in portrait orientation, watch your shadow and feet - they might also make a guest appearance!
If you do get one, make sure to practice before you use it to get used to the depth with leading lines to your advantage before going away - then it can be a very useful lens for you.
Here are a couple of sigma 10-20 examples:
@ 10mm (and about 12" from the nearest stones)