Only Online UPS are quite good.
I have had a APC UPS of 2002 make which did little in AVR,all it did was when the voltage goes to around 260v it goes to back up mode.
My new APC 1000UXI Smart UPS does AVR only after 250v.Where as my V-Guard Digi stabilizer never allows the voltage to go beyond 240v.It constantly adjust output voltage to around 215V to 230V.
So a stabilizer is more effective then a typical UPS.
Some UPS/inverter output volt upto 270V in backup mode.Also Square Wave UPS also isn't that good for any inductive electronic/electrical device these includes fans,power supply(in tv,PC...),CFL etc.