It's rated for 40g, I believe, so I would add a second filter like an AC power filter or Emperor perhaps, but if it's a good price, I would jump on it just to have on hand.
Right on with the 40g tank max. It has an advertised flow rate of 110gph but I would say that the actual output is around 95gph or so max. I agree that you would want another filter to aid in filtration. If the deal is that good pick up two of them.
For $30 it might not be a bad deal. I had a bad experience with the Fluval x04 series, so I personally don't recommend them to people, but other people like them and $30 is cheap. I wouldn't use it as the only filter on a 45g though.
You might also look at these Sun Sun filter things that people are talking about; I haven't bought one but they look awfully similar to the Marineland C series and they're relatively inexpensive too.
110 gph is nowhere near enough for a 45 gallon tank, canister or not. If you want to stick with canisters, you'll need something at least three times as powerful. 110 gph is going to seem nearly stagnant and the fish will behave like it