For something cheap, cheerful and effective, give Toneboosters 'Reelbus' a spin.

Or NomadFactory Magnetic II, also a nice one.

I just use the Tape Sim PC module in SONAR.   
 
However, I don't use it on everything, just tracks I think may benefit in the context of the song; e.g., maybe bass, organ or electric guitar, but rarely acoustic piano or vocals where I want to retain the best clarity and definition that I can.

I sometimes add tape hiss to the preroll for a tune.

To sum up what I've just read here
1. Two basic approaches
a) TE on every track.  It resembles a practice to record first and then mix off the record.
b) TE on every bus. Opposite to the above - mix first and then print to tape.
2. Always, every time. Seems like once you try a tape emulator, you never look back. But why?
3. Sounds nice, adds color and punch.  Nobody mentioned that tape emu is the easiest way to add subtle compression, saturation and color to you mix. But what if in the end, when you think your mix sounds awesome, you turn TE off and suddenly, as smallstonefan said,

smallstonefan
The stereo field opened up considerably and things sounded more open.

The main questions WHY and WHEN are still open.
I started recording when I was a teen. I had a consumer grade tube tape deck. It wasn't great, I wished it had some more clarity, headroom, i.e. better sound quality. Yeah, it wasn't even close to top-notch studio decks of the 80's which are modeled with contemporary technology.
Yet, correct me if I'm wrong, isn't TE just a custom built MB compression + saturation (+noise generator/IR)?
What is the main reason to use a TE, besides mimicking great records of the past and replacing compression/saturation routine?

I am a member of the following clans:
 
1) just because you can, doesn't mean you should
 
2) ASSAP (As Stupidly Simple As Possible)
 
3) sear in the rear (works for steaks as well as mixes)
 
I doesn't make sense to me to add a bunch of audio content (via TEs, in this case) and then get to the master buss and find out I've got to go back and make more space for an instrument or vocal. As simple and clean to the busses and master buss as possible is how I prefer to roll. Once I like a mix, THEN I can start messing with the sonic nature of the whole.
 
It also doesn't make sense to me to spend a bunch of money on good preamps, and then the very first thing you do is distort the signal with a TE.
 
Caveat: I don't have the same positive historical relationship with recording to tape many are mentioning. I had an Asian model JVC reel-to-reel that I bought in Germany in 1971 that I liked a lot, but it went missing when I got shipped back to the States and discharged on a medical. I wound up with an 8-track system, and yes, they were a beatch to record to. When cassettes came out, I switched to them. Never got my hands on a big tape deck again. Sad.

Sound. You might like the sound you can create with it. Really, that is the point of all plugins.

gswitz
Sound. You might like the sound you can create with it. Really, that is the point of all plugins.

Fleer
Or NomadFactory Magnetic II, also a nice one.

I quite agree.  A lot of folks seem to regard Magnetic II as second rate compared to the bigger name brands (UAD, Slate, etc.).  But I quite like it when used as a master bus effect and used with restraint.

if i use something like tape saturation (which is really just a specialized distortion), i do it in parallel.
 
push it in as hard as i want, then mix it back with the pure tracks via sends.