I have a character who uses shield, and I am not told the number rolled (because we play through a VTT and it would slow the game down for the DM to make dice cups for all the opponents, when he can just quickly roll physical dice). In no way do I feel the spell is underpowered or I am cheated by not knowing for certain if the spell will block the triggering attack. I mean, it gives me a +5 AC for a round, and I don't need to use it until I think there is a good chance it will help...and then it keeps on helping against any other attacks.
It seems to me that granting certain knowledge of whether or not the spell will block the attack is turning an already excellent spell (almost a must have spell) into a ridiculously good spell.
If you are only ever targeted by one attack per round, then I suppose you might not feel the spell is that amazing at lowish (but above 1st-2nd) levels, but once you gain a few levels, even if you only are targeted by a single attack, a solid chance to block a nasty slap of damage at the cost of only a 1st level slot and a reaction starts looking really good.