Is there a way to force RSpec to re-evaluate a let statement?

You can just define a regular method:

def number_of_users
  User.count
end

it 'counts users' do
  User.create
  number_of_users.should == 1
  User.create
  number_of_users.should == 2
end

See this blog post for some more details, including how to store the helper methods in a separate module.

How about writing such test like this:

it "counts user" do
  expect {
    User.create
  }.to change(User, :count).by(1)
end