Lissandra's buffs are interesting, but I think a different direction is needed.
Luckily Rickless agrees and will likely pull back the R buff, but let's talk a bit more about Lissandra.
The intended buff for R would give Lissandra back 15% of her missing Mana on self-cast. This means Riot understands there is a problem with Lissandra's Mana regeneration, but I don't think encouraging her self-casts for ultimate are really going to be the saving grace.
Let's review the Mana numbers:
At level 1, Lissandra's base Mana regen is 5.67. Taric's is 5. These two are the lowest Mana regeneration champions at level 1 in the game. Lissandra's Mana regen per level is 0.4, one of the lowest numbers in the game; most others with 0.4 Mana regen are Supports and ADCs. Taric's is 1. This means that at level 2, Lissandra has 0.07 more Mana regen than Taric, and Taric beats Lissandra from level 3 onward.
At level 18, Lissandra's value is 12.5. This is a number she shares with Darius, and together they have the lowest Mana regeneration at level 18.
Among Mid laners, Lissandra has the unquestionable lowest Mana regeneration rate in the entire game.
Her base Mana value is also one of the lowest for Mid lane: the only Mid laners with less base Mana are Diana, Gangplank and Twisted Fate. She has exactly 4 more base Mana than Illaoi.
At 85 Mana (Q), 50 Mana (W) and 80-100 Mana (E), Lissandra's costs are also rather high. At level 3, she has 378 Mana (without items/masteries/runes), meaning a single cast of Q consumes around ~22% of her Mana pool.
To put that into perspective: Lux is also considered a champion with high Mana costs, and at level 3 she has 453 Mana. Casting a single E sets her back 70 (rank 1) or 85 (rank 2), which is always a lower percentage of her Mana pool compared to Lissandra.
I think it's great that Riot recognises Lissandra has Mana issues, but I don't think the fix is in the R. I think a good fix would be to look at her early Mana regen numbers. I get that she has that Passive that lets her cast for free, but it's insufficient to let Lissandra deal with strong pushers. Releasing the tight reigns on her Mana regeneration/Mana pool numbers might free her up to push harder and perform her famous roam plays a bit more, or at least give her a sporting chance against heavy pushers in the early game.
Usually I wind up buying 
to fix her early Mana problems, but since the nerf to Lissandra's maximum health there are only eight champions with less health at level 18. I understand she was considered too tanky, but it means Lissandra's close-range is extremely dangerous to her and she needs to invest in Health or defensive items not only because of playstyle but also because her base values force that.
That's a slight tangent, but I think her health could be a valuable place for a small buff as well.
tl;dr the Q buff is good, but I think rather than the planned buff to R we're better suited with a (small) Mana regen buff, and maybe a buff to her health per level or Mana pool.
