For each real $k>0$, 
\begin{equation}E\psi_\infty(|X|/k)=\infty\,P(|X|>k)+P(|X|=k) \\
=\left\{\begin{aligned}\infty\text{ if } P(|X|>k)>0,\\ 
P(|X|=k)\le1\text{ if } P(|X|>k)=0.
\end{aligned}\right.
\end{equation}
So, indeed, $\|X\|_{\psi_\infty}<\infty$ iff $X$ is essentially bounded. Moreover, $\|X\|_{\psi_\infty}=\text{ess}\,\text{sup}\,|X|$. 

Generally, for any nondecreasing convex function $F\colon[0,\infty)\to[0,\infty]$ such that $F(x)\to0$ as $x\downarrow0$ and $F(x)\to\infty$ as $x\to\infty$, the formula 
\begin{equation}\|X\|_F:=\inf\{t>0\colon EF(|X|/t)\le1\}
\end{equation}
defines a norm on the linear space of random variables $X$ on a probability space with $\|X\|_F<\infty$. The proof of this is the same as the one in the case when $F$ is not allowed to take the value $\infty$.