The I-4 engine fires every 180 degrees. For instance, #1 is firing when #4 is filling. Likewise it suffers from free forces of the second order (inertial force dependant on rotating mass,crank radius, rod/throw ratio and 2xcrank angle); translation, the engine wants to shake up and down. Some manufacturers use counterbalance shafts to help with these forces. But ultimately it is not a very smooth engine.

I-5 engines fire every 144 degrees. That is the main reason why they are smoother. However I-5's suffer from free moments of the first and second order. This means the engine wants to rotate about the centerline of the car and also rotate about the vertical axis. If large enough the free moments will rock the body of the car from side-to-side, full size pick-up trucks do this alot when you hit the gas in neutral. This is why many people (myself included) buy a replacement torque bushing for the upper engine attachment. The free moments rip the bushing apart as it ages and becomes brittle.

I-6 engines (flat 6 in Porsches too) fire every 120 degrees and do not suffer from any inertial problems. It is an inherently balanced design.

V-6 engines come in two angles, 60 and 90 degrees. The 90 degree variant is made on the same assembly line as V-8 engines. 90 degree engines fire at 150/90 degrees. This means 1 cylinder fires at 150 degrees of crank rotation then the next 90 degrees later (repeat twice). Like the I-5 it suffers from first and second order free moments. So it should run about as smoothly as an I-5. Aside: I think Mercedes uses a counterbalance shaft to absorb/reduce the effects of these free moments. The 60 degree v-6 fires every 120 degrees. Based on this it should give power delivery similarly to the I-6. But this v-6 suffers from first and second order free moments (much bigger than all of the other mentioned previously).

As a final note: my fathers 15 year old BMW with over 400,000 miles is smoother than the I-4 Saab I bought when the saab was new. Of course the 850 was smoother than the BMW but not much.