A downhole tool useable for isolating sections of a wellbore, the downhole tool having a longitudinal axis and comprising: a mandrel (214, 314) made of a composite material, the mandrel comprising: a distal end; a proximate end; and a bore (250, 350) formed therethrough; wherein the mandrel is provided with a first set of threads which are internal threads (216, 316) within the bore (250, 350) and which are configured to receive a mechanical or wireline setting tool or adapter, so as to couple the mandrel (214, 314) with corresponding threads of said mechanical or wireline setting tool or adapter, wherein the first set of threads are shear threads (216, 316) disposed along an inner surface of the bore (250, 350) at the proximate end, such that application of a load to the mandrel (214, 314) can shear the first set of threads (216, 316); and wherein the mandrel is further provided with a second set of threads which are rounded threads (218, 318) having a round thread profile, said rounded threads (218, 318) being disposed on an external mandrel surface at the distal end and being configured to mate with a lower sleeve (260, 360) having corresponding threads, so that the mandrel (214, 314) may be coupled with said lower sleeve (260, 360), whereby said rounded threads (218, 318) allow a non-axial interaction between mating surfaces, such that there are vector forces in directions other than the shear direction and the round thread profile creates radial load instead of shear, meaning that setting of the tool distributes load forces along the rounded threads at an angle that is directed away from the axis of the tool; a composite member (220, 320) disposed about the mandrel wherein the composite member (220, 230) is made of a first material and comprises a deformable portion and a resilient portion, wherein the resilient portion is configured with increased resilience to deformation as compared to the deformable portion, such that upon application of a load, the resilient portion will withstand the load but the deformable portion will change shape from its original configuration and/or position; and a seal element disposed about the mandrel, wherein the composite member (220, 320) is in engagement with the seal element; and a one-piece slip (234, 242) disposed about the mandrel. a mandrel (214, 314) made of a composite material, the mandrel comprising: a distal end; a proximate end; and a bore (250, 350) formed therethrough; wherein the mandrel is provided with a first set of threads which are internal threads (216, 316) within the bore (250, 350) and which are configured to receive a mechanical or wireline setting tool or adapter, so as to couple the mandrel (214, 314) with corresponding threads of said mechanical or wireline setting tool or adapter, wherein the first set of threads are shear threads (216, 316) disposed along an inner surface of the bore (250, 350) at the proximate end, such that application of a load to the mandrel (214, 314) can shear the first set of threads (216, 316); and wherein the mandrel is further provided with a second set of threads which are rounded threads (218, 318) having a round thread profile, said rounded threads (218, 318) being disposed on an external mandrel surface at the distal end and being configured to mate with a lower sleeve (260, 360) having corresponding threads, so that the mandrel (214, 314) may be coupled with said lower sleeve (260, 360), whereby said rounded threads (218, 318) allow a non-axial interaction between mating surfaces, such that there are vector forces in directions other than the shear direction and the round thread profile creates radial load instead of shear, meaning that setting of the tool distributes load forces along the rounded threads at an angle that is directed away from the axis of the tool; a composite member (220, 320) disposed about the mandrel wherein the composite member (220, 230) is made of a first material and comprises a deformable portion and a resilient portion, wherein the resilient portion is configured with increased resilience to deformation as compared to the deformable portion, such that upon application of a load, the resilient portion will withstand the load but the deformable portion will change shape from its original configuration and/or position; and a seal element disposed about the mandrel, wherein the composite member (220, 320) is in engagement with the seal element; and a one-piece slip (234, 242) disposed about the mandrel.