I'll hop in here before the Lana Haterz take over.
Speaking as a big fan of KK and Lana, I do miss her presence on this show, just as I miss MR/Lex.
But... I also understand that story-wise, after 8 seasons of Lana and Clark being used as a source of doomed angst, the show HAD to move on. Once they cast a Lois Lane on this show, and continued to have her as a regular, Lana's days were numbered. That's just common sense for a show based on an established mythology that is working towards embracing that mythology more than ever in this possibly last season.
So, yes, I do miss her presence on this show, a lot even, it feels like a different show to me this season without Lex and Lana. Fortunately, I think they've done wonders with Erica's Lois Lane and her interactions with Clark. The show needed a heavy dose of "fun" and those two are always entertaining to watch. I'm not one who really feels a lot of romantic chemistry between them, but I like Lois enough to be able to roll with it anyway. Erica has really stepped up to the plate and hit a home run in terms of embracing this role in a much more layered way than she was initially. I love her serious and thoughtful moments with Clark, more than her snark, really. I'm glad they are showing us more and more of that. It goes well towards establishing a believable relationship between these two people as "soulmates" in the future.
I'm kind of dreading Lana's return, not because of the expected rantings from those who consider her a threat to the new direction of the show, but because I'm worried the writers will fall into their old habits of using her inconsistently as a story device, as opposed to a consistent characterization. For instance, I don't want to see 5 episodes of Lana showing us ONCE AGAIN, why she and Clark aren't *meant to be*. WE GET IT ALREADY. I think the new powers have done a bang-up job so far of progressing all the characters, and I hope they stick to that formula for Lana Lang, as opposed to dragging her back to square one.
SV fans have always defended SV's penchant for taking the mythology and putting their own twist on it, and I'm really hoping that they won't put Lana and Lois in a cliche' catfight situation over Clark. If anything, SV has established several times that Lana is willing to sacrifice what she wants (Clark) for what's best for Clark and his destiny. I'm hoping that there's a continued acknowledgment of that from her - even in regards to Lois - before she leaves.
I do think that for losing 2 of their original 3 leads, the show has done an amazing job so far.
But yeah... the chemistry between TW and KK is, and always was, incomparable to any other on the show. Those SV folks have a fantastic track record when it comes to casting for this show.
No matter what the many *edit* nay sayers on here say, Kreuk has a sweet quality to her acting and imo 10x the chemistry with Tom than Erica (blech). 
