Yes, it's plasticky and cheap in terms of ergonomics and construction quality. No USM motor, and autofocus is slower than I'd like (this only really matters on the longer end of the 250mm zoom.)
But optically, this lens is excellent -- it's sharp at both ends of the zoom. I used to have the 70-200mm F4 L (non-IS), and I can safely say this cheap zoom (55-250) matches or exceeds the F4's sharpness. (Note, I'm talking about the older non-IS 70-200mm F4, not the newer IS version.)
The 55-250 has superior zoom-range, but inferior autofocus. On my Rebel XSi, it's not very good at focus-tracking moving subjects, and it does tend to hunt/wander in low-light (especially at the 250mm end.) Perhaps on a more expensive body (like the 40D/50D), it would autofocus better.
In summary, the optical performance is solid, daresay excellent for the price. (It goes well with the 18-55mm F4-5.6 IS) If it breaks because of the cheap build quality, just buy another...