I use iTeleport for this--it's awesome.
My uses:
* My iPhone (and soon, iPad) are my "sofa remote" when I don't want to lug a BT mouse and keyboard. (My iMac is my TV so this arises often. For instance, I can navigate Hulu with iTeleport.) For this purpose I often disable the screen display and just look at the Mac (saves battery and network bandwidth too).
* With the iPhone's screen disabled, you can pinch zoom the Mac's OWN screen using the iPhone. Very nice!
* Emergency access to my business files/apps/records on my desktop when I'm out. (I leave my home Mac on when I'm expecting an important call that may relate to those files.) iTeleport is slow via cell network, but it works! Even over Edge if you're the patient type...
* Setting the Mac to turn on for a short time every day at the same time, so when I'm traveling I know I can get on at that time (and stay on as long as I need). This is in Energy Saver prefs on the Mac.
* Telling my desktop Mac to email a file to my laptop while on the road. (I can then receive it once my laptop gets to a hotspot.)
* Accessing my Time Capsule remotely to see a complete history of files/versions on ALL my Macs (by connecting via my iMac). For instance, sometimes I want to restore an old file to my laptop... but I'm not at home to access the Time Capsule.
* Going to bed with my Mac still doing something (like a big FTP transfer) that I want to keep an eye on from bed (and then sleep the Mac when done).
* Checking on a Mac in another room without spending the 3 calories to walk there
* Presenting stuff on my MacBook Air: I turn the laptop away from me to face the audience (small meetings) and control it from my iPhone. Or, if there's a TV/projector around, I hook the Air to that and my iPhone makes a great controller with full mouse/scrollwheel/keyboard.
* Light Flash use occasionally, if a client wants to discuss some Flash site while I'm away from my Mac.
* Remote tech support for friends/family who let me view their screens. I can see their screen from my iPhone (takes some pre-configuring on their end) and then advise them. (Could be done with Windows, but I've only tried with Mac.)
* Viewing my apartment when I'm traveling: I turn on PhotoBooth for a look around. If I'm having work done, I can even turn the Mac to face the kitchen (or wherever it is) before I go.
(You can do a lot of this just a laptop, viewing the remote computer--and I often do, via Apple Screen Sharing, which is iTeleport-compatible standard VNC anyway. But when the laptop has no Internet--or when I didn't lug a laptop at all--the iPhone is great.)
iTeleport is the most expensive app I ever bought, but worth it. The precision of the mouse control (with Momentum off, I recommend) and the many zoom and keyboard options are just great. Initial config was hairy, with my router and everything, but I got it working. (Now it supports more automated VNC config methods like Google, which I haven't had to try yet.)
I can't compare speed of iTeleport to others, but I can say that on LAN it's pretty quick unless moving a huge window (noticeable lag but not a problem). I wouldn't watch video that way (there are better ways) and it has no audio.