I personally like them. They aren't Vanguard. You have to play them just like any other spell. If anything, I think their restrictions are going to keep them from seeing serious play anyway. Consider: They are legendary by their very nature and they can be countered, just like any other spell. That's a couple of huge strikes against them.
They are also quite vulnerable because they can be attacked directly. Fortunately, you can block when that happens, but even a 1/1 hitting one can have a significant impact. Realistically, in many circumstances, Garrak is a 3/3 creature for 4 with a built in "kill 1 2/2 and force another 2/2 (or 3/3) to skip an attack and block.
The one advantage this has over other legendaries is that if you happen to draw a second, you can run the first one out of loyalty and then play the second one fresh. In fact, that seems kind of cludgy to me, to be honest - you can play another and get the affect a second time in the same turn, but only if the first use of the turn exhasts him completely...
I think these will only work with a defensive creature strategy. Consider Garruk - get a bunch of saporlings into play so that if someone wants to attack him when you first put him into play, you can deal with it. You get extra mana for a few turns, then you have an army of 4/4 trampling saporlings... In a pinch, you drop a 3/3 beast or two to help hold off the opponent.
Liliana is potentially more powerful, but probably way more fragile. Still, if all she does is force your opponent to discard 2-3 cards... Personally, they are both likely too slow to be real contenders, but we don't know how the rest of the set plays out. There are probably supporting cards to add loyalty...
