I personally like the concept of Rylais crystal scepter, but I dont think it is worth the money and needs a slight buff.
The issue I would like to adress is the ineffectiveness of the slow. Due to the nature of mages, most of their kit is AoE, and AoE abilities apply a 15% slow for 1.5 seconds. This just seems underwhelming. I suggest increasing the effectiveness of AoE abilities to a 20-25% slow.
Furthermore, lets look at the stats. Compared to a fully stacked Rod of Ages, it gives 150 less health, 650 less mana, and costs 100 more in exchange for a slow (plus RoA has sustain from Chalice). This just seems a little weak.