D.I
Basically, correct me if I'm wrong, D.Is boost your sound signal, which is absolutely essentially if you play in live locations. e.g church, bars and clubs, etc. Essentially, D.Is are not effects, though some, like the MXR D.I, has built-in effects. Usually, such built-in effects are overdrives or distortions. The difference, my guessing, is that the main function is to shape your sound (e.g Lows, Mids, Highs, and the in-betweens).
My take is, if you are looking for a GOOD/SOLID sound driver/distortion, you should get a standalone pedal rather than depend on the D.I's built-in effect.
And on the other hand, if you play alot in a live setting, especially in church, I highly recommend a good separate D.I box. This will reduce the loss of tone (the signal output from your instrument is not enough to go through a long circuit length, though D.Is are not necessary if you are just playing with a direct signal to an amp) as your signal goes through a long series of circuits before the final output through the house speakers. I'm currently using the Radial Tonebone Bassbone as my D.I box.
Anyone else wants to add on?