I own a Santro (not the eRLX) and my friend owns a Swift. I drive his car a lot, almost as much as I drive mine.
First of all, it is not fair to compare a Swift with a Santro. Santro is a wonderful car no doubt, but they belong to different segments. Swift is bigger and is powerful than Santro. Ofcourse, if you are thinking just about the cost/value ratio, then Santro is better, but otherwise...Getz is the alternative for Swift, not Santro.
Coming to the original question
Swift is a nice car. Great looks, good to drive, comfortable, a proven engine and ofcourse a very attractive sticker price.
So, what are the things I think as the issues with Swift?
i) Boot space is low
ii) The sloped dashboard
iii) The interiors look cheap
iv) Rear legspace is less
v) Sitting at the backseat is like sitting inside a cave (thanks to the massive C-pillar & sloping windows)
vi) The gear shaft is typical Maruti - not smooth.
My friend's car runs 12 kms / litre - with AC switched on. Not great, but still okay in Bangalore traffic.
As you can see, the issues are minor but many of them are subjective. If you are okay with them...go and grab one.
Hope that helps.