I can't say I've ever read any of the Mass Effect books haha x
I haven't read Deception, thanks to the rightful negative feedback promptly stopping me from ordering it (I was already apprehensive when discovering that Drew Karpyshyn hadn't written it).
I've read Revelations, Ascension and Retribution all thrice now (I usually reread all books I own) and personally, Ascension is my favourite, with Retribution being the weakest for me.
If you love Mass Effect, and generally like expanded universe material like comics and the like, the books are worth a go. It has to take a game series I really enjoy for me to look into the other media and merchandise that has been made.
They aren't excellent, and Drew's prose as previously mentioned here isn't something I'd peg as particularly strong (I actually cringed when he used hamburger as an adjective), but if you love Mass Effect then they are light, fluffy enjoyable fun, and they were good enough for me that I reread them. But it is down to everyone's personal taste.
I'd say the biggest draw is that it features a strong likable protagonist in Kahlee Sanders, who was woefully underused in her brief appearance in Mass Effect 3.